diff --git a/library/think/Session.php b/library/think/Session.php
index adf93441..ed0d0b57 100644
--- a/library/think/Session.php
+++ b/library/think/Session.php
@@ -56,7 +56,7 @@ class Session
             $isDoStart = true;
         }
 
-        if (isset($config['prefix']) && self::$prefix === '') {
+        if (isset($config['prefix']) && self::$prefix === '' && self::$prefix === null) {
             self::$prefix = $config['prefix'];
         }
         if (isset($config['var_session_id']) && isset($_REQUEST[$config['var_session_id']])) {