diff --git a/cmd/explaintest/r/new_character_set_builtin.result b/cmd/explaintest/r/new_character_set_builtin.result
index b4944f3057611..8772baab07111 100644
Binary files a/cmd/explaintest/r/new_character_set_builtin.result and b/cmd/explaintest/r/new_character_set_builtin.result differ
diff --git a/cmd/explaintest/t/new_character_set_builtin.test b/cmd/explaintest/t/new_character_set_builtin.test
index 0c6fefac7919d..3726951591461 100644
--- a/cmd/explaintest/t/new_character_set_builtin.test
+++ b/cmd/explaintest/t/new_character_set_builtin.test
@@ -57,6 +57,13 @@ select hex(convert('ㅂ' using gbk)), convert('ㅂ' using gbk);
 select convert(a using binary) from t;
 select convert(convert('中文' using gbk) using binary), convert('中文' using binary);
 select convert(convert('ㅂ' using gbk) using binary), convert('ㅂ' using binary);
+drop table if exists t;
+create table t(a binary(10));
+insert into t values (0xe240), (0x01e240);
+set @@tidb_enable_vectorized_expression = true;
+select hex(convert(a using gbk)), convert(a using gbk) from t;
+set @@tidb_enable_vectorized_expression = false;
+select hex(convert(a using gbk)), convert(a using gbk) from t;
 
 -- test for builtin function md5()
 drop table if exists t;
diff --git a/expression/builtin_string.go b/expression/builtin_string.go
index bf948605045b2..86f426be2faaa 100644
--- a/expression/builtin_string.go
+++ b/expression/builtin_string.go
@@ -41,7 +41,6 @@ import (
 	"github.com/pingcap/tidb/util/logutil"
 	"github.com/pingcap/tipb/go-tipb"
 	"go.uber.org/zap"
-	"golang.org/x/text/transform"
 )
 
 var (
@@ -1144,29 +1143,26 @@ func (b *builtinConvertSig) evalString(row chunk.Row) (string, bool, error) {
 
 	// Since charset is already validated and set from getFunction(), there's no
 	// need to get charset from args again.
-	encoding, _ := charset.Lookup(b.tp.Charset)
+	enc := charset.NewEncoding(b.tp.Charset)
 	// However, if `b.tp.Charset` is abnormally set to a wrong charset, we still
 	// return with error.
-	if encoding == nil {
+	if enc.Name() == "" {
 		return "", true, errUnknownCharacterSet.GenWithStackByArgs(b.tp.Charset)
 	}
 	// if expr is binary string and convert meet error, we should return NULL.
 	if types.IsBinaryStr(b.args[0].GetType()) {
-		target, _, err := transform.String(encoding.NewEncoder(), expr)
+		target, err := enc.DecodeString(expr)
 		if err != nil {
-			return "", true, err
+			b.ctx.GetSessionVars().StmtCtx.AppendWarning(err)
+			return "", true, nil
 		}
-
-		// we should convert target into utf8 internal.
-		exprInternal, _, _ := transform.String(encoding.NewDecoder(), target)
-		return exprInternal, false, nil
+		return target, false, nil
 	}
 	if types.IsBinaryStr(b.tp) {
 		enc := charset.NewEncoding(b.args[0].GetType().Charset)
 		expr, err = enc.EncodeString(expr)
 		return expr, false, err
 	}
-	enc := charset.NewEncoding(b.tp.Charset)
 	return string(enc.EncodeInternal(nil, []byte(expr))), false, nil
 }
 
diff --git a/expression/builtin_string_test.go b/expression/builtin_string_test.go
index 28d98d0215091..22f3ea294d129 100644
--- a/expression/builtin_string_test.go
+++ b/expression/builtin_string_test.go
@@ -885,14 +885,16 @@ func TestConvert(t *testing.T) {
 		str           interface{}
 		cs            string
 		result        string
+		isNull        bool
 		hasBinaryFlag bool
 	}{
-		{"haha", "utf8", "haha", false},
-		{"haha", "ascii", "haha", false},
-		{"haha", "binary", "haha", true},
-		{"haha", "bInAry", "haha", true},
-		{types.NewBinaryLiteralFromUint(0x7e, -1), "BiNarY", "~", true},
-		{types.NewBinaryLiteralFromUint(0xe4b8ade696870a, -1), "uTf8", "中文\n", false},
+		{"haha", "utf8", "haha", false, false},
+		{"haha", "ascii", "haha", false, false},
+		{"haha", "binary", "haha", false, true},
+		{"haha", "bInAry", "haha", false, true},
+		{types.NewBinaryLiteralFromUint(0x7e, -1), "BiNarY", "~", true, true},
+		{types.NewBinaryLiteralFromUint(0x1e240, -1), "utf8", "", true, false},
+		{types.NewBinaryLiteralFromUint(0xe4b8ade696870a, -1), "uTf8", "中文\n", true, false},
 	}
 	for _, v := range tbl {
 		fc := funcs[ast.Convert]
@@ -908,8 +910,12 @@ func TestConvert(t *testing.T) {
 
 		r, err := evalBuiltinFunc(f, chunk.Row{})
 		require.NoError(t, err)
-		require.Equal(t, types.KindString, r.Kind())
-		require.Equal(t, v.result, r.GetString())
+		if !v.isNull {
+			require.Equal(t, types.KindString, r.Kind())
+			require.Equal(t, v.result, r.GetString())
+		} else {
+			require.True(t, v.isNull)
+		}
 	}
 
 	// Test case for getFunction() error
@@ -1441,9 +1447,8 @@ func TestChar(t *testing.T) {
 		{"65", 16740, 67.5, "utf8", "AAdD", 0},           // large num
 		{"65", -1, 67.5, nil, "A\xff\xff\xff\xffD", 0},   // nagtive int
 		{"a", -1, 67.5, nil, "\x00\xff\xff\xff\xffD", 0}, // invalid 'a'
-		// TODO: Uncomment it when issue #29685 be closed
-		// {"65", -1, 67.5, "utf8", nil, 1},                 // with utf8, return nil
-		// {"a", -1, 67.5, "utf8", nil, 2},                  // with utf8, return nil
+		{"65", -1, 67.5, "utf8", nil, 1},                 // with utf8, return nil
+		{"a", -1, 67.5, "utf8", nil, 2},                  // with utf8, return nil
 		// TODO: Uncomment it when gbk be added into charsetInfos
 		// {"1234567", 1234567, 1234567, "gbk", "謬謬謬", 0},  // test char for gbk
 		// {"123456789", 123456789, 123456789, "gbk", nil, 3}, // invalid 123456789 in gbk
diff --git a/expression/builtin_string_vec.go b/expression/builtin_string_vec.go
index 11933f305fe58..7612836bf21dc 100644
--- a/expression/builtin_string_vec.go
+++ b/expression/builtin_string_vec.go
@@ -30,7 +30,6 @@ import (
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/util/chunk"
 	"github.com/pingcap/tidb/util/collate"
-	"golang.org/x/text/transform"
 )
 
 func (b *builtinLowerSig) vecEvalString(input *chunk.Chunk, result *chunk.Column) error {
@@ -679,17 +678,14 @@ func (b *builtinConvertSig) vecEvalString(input *chunk.Chunk, result *chunk.Colu
 	}
 	// Since charset is already validated and set from getFunction(), there's no
 	// need to get charset from args again.
-	encoding, _ := charset.Lookup(b.tp.Charset)
+	enc := charset.NewEncoding(b.tp.Charset)
 	// However, if `b.tp.Charset` is abnormally set to a wrong charset, we still
 	// return with error.
-	if encoding == nil {
+	if enc.Name() == "" {
 		return errUnknownCharacterSet.GenWithStackByArgs(b.tp.Charset)
 	}
-	encoder := encoding.NewEncoder()
-	decoder := encoding.NewDecoder()
 	isBinaryStr := types.IsBinaryStr(b.args[0].GetType())
 	isRetBinary := types.IsBinaryStr(b.tp)
-	enc := charset.NewEncoding(b.tp.Charset)
 	if isRetBinary {
 		enc = charset.NewEncoding(b.args[0].GetType().Charset)
 	}
@@ -702,13 +698,12 @@ func (b *builtinConvertSig) vecEvalString(input *chunk.Chunk, result *chunk.Colu
 		}
 		exprI := expr.GetString(i)
 		if isBinaryStr {
-			target, _, err := transform.String(encoder, exprI)
+			target, err := enc.DecodeString(exprI)
 			if err != nil {
-				return err
+				result.AppendNull()
+				continue
 			}
-			// we should convert target into utf8 internal.
-			exprInternal, _, _ := transform.String(decoder, target)
-			result.AppendString(exprInternal)
+			result.AppendString(target)
 		} else {
 			if isRetBinary {
 				str, err := enc.EncodeString(exprI)
diff --git a/expression/integration_test.go b/expression/integration_test.go
index d25e3ad04195a..f0d949cf5735f 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -10580,3 +10580,15 @@ func (s *testIntegrationSuite) TestIssue29244(c *C) {
 	tk.MustExec("set tidb_enable_vectorized_expression = off;")
 	tk.MustQuery("select microsecond(a) from t;").Check(testkit.Rows("123500", "123500"))
 }
+
+func (s *testIntegrationSuite) TestIssue29685(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t (a binary(5));")
+	tk.MustExec("insert into t values (0x1e240), ('ABCDE');")
+	tk.MustExec("set tidb_enable_vectorized_expression = on;")
+	tk.MustQuery("select convert(t.a using utf8) from t;").Check(testkit.Rows("<nil>", "ABCDE"))
+	tk.MustExec("set tidb_enable_vectorized_expression = off;")
+	tk.MustQuery("select convert(t.a using utf8) from t;").Check(testkit.Rows("<nil>", "ABCDE"))
+}
diff --git a/parser/charset/encoding.go b/parser/charset/encoding.go
index 72bff5b6cf7e9..9f9302a6e5ac0 100644
--- a/parser/charset/encoding.go
+++ b/parser/charset/encoding.go
@@ -19,6 +19,7 @@ import (
 	"reflect"
 	"strings"
 	"unicode"
+	"unicode/utf8"
 	"unsafe"
 
 	"github.com/cznic/mathutil"
@@ -140,8 +141,40 @@ func (e *Encoding) EncodeInternal(dest, src []byte) []byte {
 	return dest
 }
 
+// validUTF8 checks whether there are illegal utf8 characters in []byte.
+func (e *Encoding) validUTF8(src []byte) ([]byte, error) {
+	resultBytes := src
+	for len(src) > 0 {
+		r, size := utf8.DecodeRune(src)
+		if r == utf8.RuneError && size == 1 {
+			return resultBytes, e.generateErr(src, characterLengthUTF8(src))
+		}
+		src = src[size:]
+	}
+
+	return resultBytes, nil
+}
+
+// validUTF8String checks whether there are illegal utf8 characters in string.
+func (e *Encoding) validUTF8String(src string) (string, error) {
+	resultStr := src
+	for len(src) > 0 {
+		r, size := utf8.DecodeRuneInString(src)
+		if r == utf8.RuneError && size == 1 {
+			srcBytes := []byte(src)
+			return resultStr, e.generateErr(srcBytes, characterLengthUTF8(srcBytes))
+		}
+		src = src[size:]
+	}
+
+	return resultStr, nil
+}
+
 // Decode convert bytes from a specific charset to utf-8 charset.
 func (e *Encoding) Decode(dest, src []byte) ([]byte, error) {
+	if e.name == encodings[CharsetUTF8].name {
+		return e.validUTF8(src)
+	}
 	if !e.enabled() {
 		return src, nil
 	}
@@ -150,6 +183,9 @@ func (e *Encoding) Decode(dest, src []byte) ([]byte, error) {
 
 // DecodeString convert a string from a specific charset to utf-8 charset.
 func (e *Encoding) DecodeString(src string) (string, error) {
+	if e.name == encodings[CharsetUTF8].name {
+		return e.validUTF8String(src)
+	}
 	if !e.enabled() {
 		return src, nil
 	}