diff --git a/expression/expr_to_pb_test.go b/expression/expr_to_pb_test.go
index 66f07e4d7846e..c7b97e0227626 100644
--- a/expression/expr_to_pb_test.go
+++ b/expression/expr_to_pb_test.go
@@ -719,6 +719,11 @@ func (s *testEvaluatorSuite) TestExprPushDownToFlash(c *C) {
 	c.Assert(err, IsNil)
 	exprs = append(exprs, function)
 
+	// CastStringAsReal
+	function, err = NewFunction(mock.NewContext(), ast.Cast, types.NewFieldType(mysql.TypeDouble), stringColumn)
+	c.Assert(err, IsNil)
+	exprs = append(exprs, function)
+
 	// Substring2ArgsUTF8
 	function, err = NewFunction(mock.NewContext(), ast.Substr, types.NewFieldType(mysql.TypeString), stringColumn, intColumn)
 	c.Assert(err, IsNil)
diff --git a/expression/expression.go b/expression/expression.go
index e4732440ef1b6..cbbc200811e23 100644
--- a/expression/expression.go
+++ b/expression/expression.go
@@ -1017,7 +1017,7 @@ func scalarExprSupportedByFlash(function *ScalarFunction) bool {
 		switch function.Function.PbCode() {
 		case tipb.ScalarFuncSig_CastIntAsInt, tipb.ScalarFuncSig_CastIntAsDecimal, tipb.ScalarFuncSig_CastIntAsString, tipb.ScalarFuncSig_CastIntAsTime,
 			tipb.ScalarFuncSig_CastRealAsInt, tipb.ScalarFuncSig_CastRealAsDecimal, tipb.ScalarFuncSig_CastRealAsString, tipb.ScalarFuncSig_CastRealAsTime,
-			tipb.ScalarFuncSig_CastStringAsInt, tipb.ScalarFuncSig_CastStringAsDecimal, tipb.ScalarFuncSig_CastStringAsString, tipb.ScalarFuncSig_CastStringAsTime,
+			tipb.ScalarFuncSig_CastStringAsInt, tipb.ScalarFuncSig_CastStringAsDecimal, tipb.ScalarFuncSig_CastStringAsString, tipb.ScalarFuncSig_CastStringAsTime, tipb.ScalarFuncSig_CastStringAsReal,
 			tipb.ScalarFuncSig_CastDecimalAsInt, tipb.ScalarFuncSig_CastDecimalAsDecimal, tipb.ScalarFuncSig_CastDecimalAsString, tipb.ScalarFuncSig_CastDecimalAsTime,
 			tipb.ScalarFuncSig_CastTimeAsInt, tipb.ScalarFuncSig_CastTimeAsDecimal, tipb.ScalarFuncSig_CastTimeAsTime, tipb.ScalarFuncSig_CastIntAsReal, tipb.ScalarFuncSig_CastRealAsReal:
 			return true