diff --git a/executor/insert_test.go b/executor/insert_test.go
index 591f141aaba58..891728c2d050f 100644
--- a/executor/insert_test.go
+++ b/executor/insert_test.go
@@ -657,3 +657,11 @@ func (s *testSuite) TestPartitionInsertOnDuplicate(c *C) {
 	tk.MustQuery(`select * from t2`).Check(testkit.Rows("1 1"))
 
 }
+
+func (s *testSuite) TestJiraIssue5366(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec(`use test`)
+	tk.MustExec(`create table bug (a varchar(100))`)
+	tk.MustExec(` insert into bug select  ifnull(JSON_UNQUOTE(JSON_EXTRACT('[{"amount":2000,"feeAmount":0,"merchantNo":"20190430140319679394","shareBizCode":"20160311162_SECOND"}]', '$[0].merchantNo')),'') merchant_no union SELECT '20180531557' merchant_no;`)
+	tk.MustQuery(`select * from bug`).Sort().Check(testkit.Rows("20180531557", "20190430140319679394"))
+}
diff --git a/expression/builtin_json.go b/expression/builtin_json.go
index 3869a1be8bcf7..3f3da54270745 100644
--- a/expression/builtin_json.go
+++ b/expression/builtin_json.go
@@ -16,6 +16,7 @@ package expression
 import (
 	"github.com/pingcap/errors"
 	"github.com/pingcap/parser/ast"
+	"github.com/pingcap/parser/mysql"
 	"github.com/pingcap/tidb/sessionctx"
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/types/json"
@@ -170,6 +171,7 @@ func (c *jsonUnquoteFunctionClass) getFunction(ctx sessionctx.Context, args []Ex
 		return nil, errors.Trace(err)
 	}
 	bf := newBaseBuiltinFuncWithTp(ctx, args, types.ETString, types.ETJson)
+	bf.tp.Flen = mysql.MaxFieldVarCharLength
 	DisableParseJSONFlag4Expr(args[0])
 	sig := &builtinJSONUnquoteSig{bf}
 	sig.setPbCode(tipb.ScalarFuncSig_JsonUnquoteSig)
diff --git a/expression/typeinfer_test.go b/expression/typeinfer_test.go
index 62aaa2d7c7075..f1a624546af5f 100644
--- a/expression/typeinfer_test.go
+++ b/expression/typeinfer_test.go
@@ -1921,7 +1921,7 @@ func (s *testInferTypeSuite) createTestCase4JSONFuncs() []typeInferTestCase {
 	return []typeInferTestCase{
 		{"json_type(c_json)", mysql.TypeVarString, charset.CharsetUTF8MB4, 0, 51, types.UnspecifiedLength},
 		// TODO: Flen of json_unquote doesn't follow MySQL now.
-		{"json_unquote(c_json)", mysql.TypeVarString, charset.CharsetUTF8MB4, 0, 0, types.UnspecifiedLength},
+		{"json_unquote(c_json)", mysql.TypeVarString, charset.CharsetUTF8MB4, 0, mysql.MaxFieldVarCharLength, types.UnspecifiedLength},
 		{"json_extract(c_json, '')", mysql.TypeJSON, charset.CharsetUTF8MB4, mysql.BinaryFlag, mysql.MaxBlobWidth, 0},
 		{"json_set(c_json, '', 0)", mysql.TypeJSON, charset.CharsetUTF8MB4, mysql.BinaryFlag, mysql.MaxBlobWidth, 0},
 		{"json_insert(c_json, '', 0)", mysql.TypeJSON, charset.CharsetUTF8MB4, mysql.BinaryFlag, mysql.MaxBlobWidth, 0},