diff --git a/executor/executor_test.go b/executor/executor_test.go
index 02feb44abaa65..d4b11b0758d8b 100644
--- a/executor/executor_test.go
+++ b/executor/executor_test.go
@@ -9686,3 +9686,29 @@ func (s *testSerialSuite) TestUnreasonablyClose(c *C) {
 	}
 	c.Assert(opsAlreadyCoveredMask, Equals, opsNeedsCoveredMask, Commentf("these operators are not covered %s", commentBuf.String()))
 }
+
+func (s *testSerialSuite) TestIssue30971(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t1, t2")
+	tk.MustExec("create table t1 (id int);")
+	tk.MustExec("create table t2 (id int, c int);")
+
+	testCases := []struct {
+		sql    string
+		fields int
+	}{
+		// Fix a bug that the column length field returned to client is incorrect using MySQL prepare protocol.
+		{"select * from t1 union select 1 from t1", 1},
+		{"select c from t2 union select * from t1", 1},
+		{"select * from t1", 1},
+		{"select * from t2 where c in (select * from t1)", 2},
+		{"insert into t1 values (?)", 0},
+		{"update t1 set id = ?", 0},
+	}
+	for _, test := range testCases {
+		_, _, fields, err := tk.Se.PrepareStmt(test.sql)
+		c.Assert(err, IsNil)
+		c.Assert(fields, HasLen, test.fields)
+	}
+}
diff --git a/executor/prepared.go b/executor/prepared.go
index e18495d45c601..0676a77215f2e 100644
--- a/executor/prepared.go
+++ b/executor/prepared.go
@@ -220,7 +220,7 @@ func (e *PrepareExec) Next(ctx context.Context, req *chunk.Chunk) error {
 	if err != nil {
 		return err
 	}
-	if _, ok := stmt.(*ast.SelectStmt); ok {
+	if p.Schema().Len() > 0 {
 		e.Fields = colNames2ResultFields(p.Schema(), p.OutputNames(), vars.CurrentDB)
 	}
 	if e.ID == 0 {