diff --git a/go.mod b/go.mod
index e654957..bacb0f7 100644
--- a/go.mod
+++ b/go.mod
@@ -8,6 +8,7 @@ require (
 	github.com/dgrijalva/jwt-go v3.2.0+incompatible
 	github.com/go-ini/ini v1.67.0
 	github.com/mitchellh/go-homedir v1.1.0
+	github.com/mozillazg/go-pinyin v0.20.0
 	github.com/mssola/user_agent v0.6.0
 	github.com/pkg/errors v0.9.1
 	github.com/spaolacci/murmur3 v1.1.0
@@ -18,7 +19,6 @@ require (
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/mozillazg/go-pinyin v0.20.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/tidwall/match v1.1.1 // indirect
 	github.com/tidwall/pretty v1.2.1 // indirect
diff --git a/string.go b/string.go
index de97f97..dd76bcb 100644
--- a/string.go
+++ b/string.go
@@ -17,6 +17,14 @@ import (
 	"github.com/axgle/mahonia"
 )
 
+var (
+	// LETTER_LIST 字母列表
+	LETTER_LIST = []string{
+		"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n",
+		"o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z",
+	}
+)
+
 // stringOperate ...
 //
 // Author : go_developer@163.com<白茶清欢>