<script>
$(document).ready(function() { 
  $('#file_upload').uploadify({ 
  'uploader' : 'uploadify/uploadify.swf', 
  'script' : 'uploadify.php',
  'folder' : 'uploads/file', 
  'formData': { 'session': '<?php echo session_id();?>'}, 
  'onComplete' : function(event, ID, fileObj, response, data) { 
   alert(response); 
  } 
  }); 
}); 
</script>

if (isset($_POST['session'])){ 
 session_id($_POST['session']); 
 session_start();//注意此函数要在session_id之后 
} 

protected void Application_BeginRequest(object sender, EventArgs e)
  {
   /* we guess at this point session is not already retrieved by application so we recreate cookie with the session id... */
   try
   {
    string session_param_name = "ASPSESSID";
    string session_cookie_name = "ASP.NET_SessionId";

    if (HttpContext.Current.Request.Form[session_param_name] != null)
    {
     UpdateCookie(session_cookie_name, HttpContext.Current.Request.Form[session_param_name]);
    }
    else if (HttpContext.Current.Request.QueryString[session_param_name] != null)
    {
     UpdateCookie(session_cookie_name, HttpContext.Current.Request.QueryString[session_param_name]);
    }
   }
   catch
   {
   }

   try
   {
    string auth_param_name = "AUTHID";
    string auth_cookie_name = FormsAuthentication.FormsCookieName;

    if (HttpContext.Current.Request.Form[auth_param_name] != null)
    {
     UpdateCookie(auth_cookie_name, HttpContext.Current.Request.Form[auth_param_name]);
    }
    else if (HttpContext.Current.Request.QueryString[auth_param_name] != null)
    {
     UpdateCookie(auth_cookie_name, HttpContext.Current.Request.QueryString[auth_param_name]);
    }

   }
   catch
   {
   }
  }

  private void UpdateCookie(string cookie_name, string cookie_value)
  {
   HttpCookie cookie = HttpContext.Current.Request.Cookies.Get(cookie_name);
   if (null == cookie)
   {
    cookie = new HttpCookie(cookie_name);
   }
   cookie.Value = cookie_value;
   HttpContext.Current.Request.Cookies.Set(cookie);
  }

<script type="text/javascript">
  var auth = "@(Request.Cookies[FormsAuthentication.FormsCookieName]==null ? string.Empty : Request.Cookies[FormsAuthentication.FormsCookieName].Value)";
  var ASPSESSID = "@Session.SessionID";

  $(function () {
   $('#upload').uploadify({
    'formData': { 'folder': '/Upload', 'ASPSESSID': ASPSESSID, 'AUTHID': auth },
    'buttonText': '浏览',
    'buttonClass': 'browser',
    'fileSizeLimit' : '100KB',
    'fileTypeExts': '*.xls;*.xlsx',
    'removeCompleted': false,
    'swf': '@Url.Content("~/Scripts/Uploadify/uploadify.swf")',
    'uploader': '/Upload',
    'onUploadSuccess': function (file, data, response) {}
   });
  });
 </script>