diff --git a/library/think/Session.php b/library/think/Session.php
index 35726220..341dce8b 100644
--- a/library/think/Session.php
+++ b/library/think/Session.php
@@ -56,8 +56,10 @@ class Session
             $isDoStart = true;
         }
 
-        if (isset($config['prefix'])) {
-            self::$prefix = $config['prefix'];
+        if (self::$prefix === '') {
+            if (isset($config['prefix'])) {
+                self::$prefix = $config['prefix'];
+            }
         }
         if (isset($config['var_session_id']) && isset($_REQUEST[$config['var_session_id']])) {
             session_id($_REQUEST[$config['var_session_id']]);