diff --git a/sm4/sm4.go b/sm4/sm4.go
index 0e301deb..edce6873 100644
--- a/sm4/sm4.go
+++ b/sm4/sm4.go
@@ -19,7 +19,6 @@ modified by Jack, 2017 Oct
 package sm4
 
 import (
-	"bytes"
 	"crypto/cipher"
 	"errors"
 	"strconv"
@@ -268,8 +267,10 @@ func xor(in, iv []byte) (out []byte) {
 
 func pkcs7Padding(src []byte) []byte {
 	padding := BlockSize - len(src)%BlockSize
-	padtext := bytes.Repeat([]byte{byte(padding)}, padding)
-	return append(src, padtext...)
+	for i := 0; i < padding; i++ {
+		src = append(src, byte(padding))
+	}
+	return src
 }
 
 func pkcs7UnPadding(src []byte) ([]byte, error) {
diff --git a/sm4/sm4_test.go b/sm4/sm4_test.go
index 6115ba14..d8b1f8a0 100644
--- a/sm4/sm4_test.go
+++ b/sm4/sm4_test.go
@@ -16,6 +16,7 @@ limitations under the License.
 package sm4
 
 import (
+	"bytes"
 	"fmt"
 	"reflect"
 	"testing"
@@ -151,3 +152,21 @@ func testCompare(key1, key2 []byte) bool {
 	}
 	return true
 }
+
+func TestPkcs7Padding(t *testing.T) {
+	src := []byte("0123456789abcdef")
+	src = pkcs7Padding(src)
+
+	want := []byte{48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 97, 98, 99, 100, 101, 102, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16}
+	if !bytes.Equal(src, want) {
+		t.Errorf("want %v, got %v", want, src)
+	}
+}
+
+func BenchmarkPkcs7Padding(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		src := []byte("0123456789abcdef")
+		src = pkcs7Padding(src)
+		_ = src
+	}
+}