diff --git a/sdk/sign.go b/sdk/sign.go
index 31863aa..5591071 100644
--- a/sdk/sign.go
+++ b/sdk/sign.go
@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"sort"
 	"strings"
+	"sync"
 )
 
 // Order represents an order object used for signing parameters
@@ -35,11 +36,18 @@ type SDK struct {
 	SecretKey string
 }
 
-// New initializes a new SDK with the given configuration
+// Singleton instance and mutex for thread safety
+var instance *SDK
+var once2 sync.Once
+
+// New initializes a new SDK with the given configuration, but ensures that only one instance exists.
 func New(secretKey string) *SDK {
-	return &SDK{
-		SecretKey: secretKey,
-	}
+	once2.Do(func() {
+		instance = &SDK{
+			SecretKey: secretKey,
+		}
+	})
+	return instance
 }
 
 // Md5String generates MD5 hash of a given string