diff --git a/executor/set_test.go b/executor/set_test.go
index 6fdb59fef08ab..601cfc9065439 100644
--- a/executor/set_test.go
+++ b/executor/set_test.go
@@ -149,48 +149,37 @@ func (s *testSuite2) TestSetVar(c *C) {
 	tk.MustExec("SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED")
 	tk.MustQuery("select @@session.tx_isolation").Check(testkit.Rows("READ-COMMITTED"))
 	tk.MustQuery("select @@session.transaction_isolation").Check(testkit.Rows("READ-COMMITTED"))
-	// error
-	_, err = tk.Exec("SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED")
-	c.Assert(terror.ErrorEqual(err, variable.ErrUnsupportedValueForVar), IsTrue, Commentf("err %v", err))
-	tk.MustQuery("select @@session.tx_isolation").Check(testkit.Rows("READ-COMMITTED"))
-	tk.MustQuery("select @@session.transaction_isolation").Check(testkit.Rows("READ-COMMITTED"))
-	// Fails
-	_, err = tk.Exec("SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE")
-	c.Assert(terror.ErrorEqual(err, variable.ErrUnsupportedValueForVar), IsTrue, Commentf("err %v", err))
-	tk.MustQuery("select @@global.tx_isolation").Check(testkit.Rows("REPEATABLE-READ"))
-	tk.MustQuery("select @@global.transaction_isolation").Check(testkit.Rows("REPEATABLE-READ"))
+	tk.MustExec("SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED")
+	tk.MustQuery("select @@session.tx_isolation").Check(testkit.Rows("READ-UNCOMMITTED"))
+	tk.MustQuery("select @@session.transaction_isolation").Check(testkit.Rows("READ-UNCOMMITTED"))
+	tk.MustExec("SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE")
+	tk.MustQuery("select @@global.tx_isolation").Check(testkit.Rows("SERIALIZABLE"))
+	tk.MustQuery("select @@global.transaction_isolation").Check(testkit.Rows("SERIALIZABLE"))
 
 	// test synonyms variables
 	tk.MustExec("SET SESSION tx_isolation = 'READ-COMMITTED'")
 	tk.MustQuery("select @@session.tx_isolation").Check(testkit.Rows("READ-COMMITTED"))
 	tk.MustQuery("select @@session.transaction_isolation").Check(testkit.Rows("READ-COMMITTED"))
 
-	_, err = tk.Exec("SET SESSION tx_isolation = 'READ-UNCOMMITTED'")
-	c.Assert(terror.ErrorEqual(err, variable.ErrUnsupportedValueForVar), IsTrue, Commentf("err %v", err))
-	tk.MustQuery("select @@session.tx_isolation").Check(testkit.Rows("READ-COMMITTED"))
-	tk.MustQuery("select @@session.transaction_isolation").Check(testkit.Rows("READ-COMMITTED"))
+	tk.MustExec("SET SESSION tx_isolation = 'READ-UNCOMMITTED'")
+	tk.MustQuery("select @@session.tx_isolation").Check(testkit.Rows("READ-UNCOMMITTED"))
+	tk.MustQuery("select @@session.transaction_isolation").Check(testkit.Rows("READ-UNCOMMITTED"))
 
-	// fails
-	_, err = tk.Exec("SET SESSION transaction_isolation = 'SERIALIZABLE'")
-	c.Assert(terror.ErrorEqual(err, variable.ErrUnsupportedValueForVar), IsTrue, Commentf("err %v", err))
-	tk.MustQuery("select @@session.tx_isolation").Check(testkit.Rows("READ-COMMITTED"))
-	tk.MustQuery("select @@session.transaction_isolation").Check(testkit.Rows("READ-COMMITTED"))
+	tk.MustExec("SET SESSION transaction_isolation = 'SERIALIZABLE'")
+	tk.MustQuery("select @@session.tx_isolation").Check(testkit.Rows("SERIALIZABLE"))
+	tk.MustQuery("select @@session.transaction_isolation").Check(testkit.Rows("SERIALIZABLE"))
 
-	// fails
-	_, err = tk.Exec("SET GLOBAL transaction_isolation = 'SERIALIZABLE'")
-	c.Assert(terror.ErrorEqual(err, variable.ErrUnsupportedValueForVar), IsTrue, Commentf("err %v", err))
-	tk.MustQuery("select @@global.tx_isolation").Check(testkit.Rows("REPEATABLE-READ"))
-	tk.MustQuery("select @@global.transaction_isolation").Check(testkit.Rows("REPEATABLE-READ"))
+	tk.MustExec("SET GLOBAL transaction_isolation = 'SERIALIZABLE'")
+	tk.MustQuery("select @@global.tx_isolation").Check(testkit.Rows("SERIALIZABLE"))
+	tk.MustQuery("select @@global.transaction_isolation").Check(testkit.Rows("SERIALIZABLE"))
 
-	_, err = tk.Exec("SET GLOBAL transaction_isolation = 'READ-UNCOMMITTED'")
-	c.Assert(terror.ErrorEqual(err, variable.ErrUnsupportedValueForVar), IsTrue, Commentf("err %v", err))
-	tk.MustQuery("select @@global.tx_isolation").Check(testkit.Rows("REPEATABLE-READ"))
-	tk.MustQuery("select @@global.transaction_isolation").Check(testkit.Rows("REPEATABLE-READ"))
+	tk.MustExec("SET GLOBAL transaction_isolation = 'READ-UNCOMMITTED'")
+	tk.MustQuery("select @@global.tx_isolation").Check(testkit.Rows("READ-UNCOMMITTED"))
+	tk.MustQuery("select @@global.transaction_isolation").Check(testkit.Rows("READ-UNCOMMITTED"))
 
-	_, err = tk.Exec("SET GLOBAL tx_isolation = 'SERIALIZABLE'")
-	c.Assert(terror.ErrorEqual(err, variable.ErrUnsupportedValueForVar), IsTrue, Commentf("err %v", err))
-	tk.MustQuery("select @@global.tx_isolation").Check(testkit.Rows("REPEATABLE-READ"))
-	tk.MustQuery("select @@global.transaction_isolation").Check(testkit.Rows("REPEATABLE-READ"))
+	tk.MustExec("SET GLOBAL tx_isolation = 'SERIALIZABLE'")
+	tk.MustQuery("select @@global.tx_isolation").Check(testkit.Rows("SERIALIZABLE"))
+	tk.MustQuery("select @@global.transaction_isolation").Check(testkit.Rows("SERIALIZABLE"))
 
 	tk.MustExec("SET SESSION tx_read_only = 1")
 	tk.MustExec("SET SESSION tx_read_only = 0")
@@ -584,8 +573,9 @@ func (s *testSuite2) TestValidateSetVar(c *C) {
 	result = tk.MustQuery("select @@tx_isolation;")
 	result.Check(testkit.Rows("REPEATABLE-READ"))
 
-	_, err = tk.Exec("set @@tx_isolation='SERIALIZABLE'")
-	c.Assert(terror.ErrorEqual(err, variable.ErrUnsupportedValueForVar), IsTrue, Commentf("err %v", err))
+	tk.MustExec("set @@tx_isolation='SERIALIZABLE'")
+	result = tk.MustQuery("select @@tx_isolation;")
+	result.Check(testkit.Rows("SERIALIZABLE"))
 }
 
 func (s *testSuite2) TestSelectGlobalVar(c *C) {
diff --git a/sessionctx/variable/session.go b/sessionctx/variable/session.go
index 936c1d4326ffd..aa07be0d44322 100644
--- a/sessionctx/variable/session.go
+++ b/sessionctx/variable/session.go
@@ -598,10 +598,6 @@ func (s *SessionVars) WithdrawAllPreparedStmt() {
 func (s *SessionVars) SetSystemVar(name string, val string) error {
 	switch name {
 	case TxnIsolationOneShot:
-		switch val {
-		case "SERIALIZABLE", "READ-UNCOMMITTED":
-			return ErrUnsupportedValueForVar.GenWithStackByArgs(name, val)
-		}
 		s.TxnIsolationLevelOneShot.State = 1
 		s.TxnIsolationLevelOneShot.Value = val
 	case TimeZone:
diff --git a/sessionctx/variable/varsutil.go b/sessionctx/variable/varsutil.go
index b4d6ddef0d70a..fc27b9c66a694 100644
--- a/sessionctx/variable/varsutil.go
+++ b/sessionctx/variable/varsutil.go
@@ -427,10 +427,6 @@ func ValidateSetSystemVar(vars *SessionVars, name string, value string) (string,
 		if !exists {
 			return "", ErrWrongValueForVar.GenWithStackByArgs(name, value)
 		}
-		switch upVal {
-		case "SERIALIZABLE", "READ-UNCOMMITTED":
-			return "", ErrUnsupportedValueForVar.GenWithStackByArgs(name, value)
-		}
 		return upVal, nil
 	case TiDBInitChunkSize:
 		v, err := strconv.Atoi(value)