diff --git a/mysql/mydecimal.go b/mysql/mydecimal.go
index 65747809fb752..cadfbfdd1f248 100644
--- a/mysql/mydecimal.go
+++ b/mysql/mydecimal.go
@@ -1512,7 +1512,7 @@ func doAdd(from1, from2, to *MyDecimal) error {
 	var x int32
 	if wordsInt1 > wordsInt2 {
 		x = from1.wordBuf[0]
-	} else if wordsInt2 < wordsInt1 {
+	} else if wordsInt2 > wordsInt1 {
 		x = from2.wordBuf[0]
 	} else {
 		x = from1.wordBuf[0] + from2.wordBuf[0]
diff --git a/mysql/mydecimal_test.go b/mysql/mydecimal_test.go
index 8fb1e9632ce19..b0a5a621aa736 100644
--- a/mysql/mydecimal_test.go
+++ b/mysql/mydecimal_test.go
@@ -14,8 +14,9 @@
 package mysql
 
 import (
-	. "github.com/pingcap/check"
 	"strings"
+
+	. "github.com/pingcap/check"
 )
 
 var _ = Suite(&testMyDecimalSuite{})
@@ -474,6 +475,7 @@ func (s *testMyDecimalSuite) TestAdd(c *C) {
 		{"123.45", "-12345", "-12221.55", nil},
 		{"-123.45", "12345", "12221.55", nil},
 		{"5", "-6.0", "-1.0", nil},
+		{"2" + strings.Repeat("1", 71), strings.Repeat("8", 81), "8888888890" + strings.Repeat("9", 71), nil},
 	}
 	for _, ca := range cases {
 		a := NewDecFromStringForTest(ca.a)