1、原来的导出按钮增加 OnClientClick事件。
        btnExport.OnClientClick = "return AutoForm_Export('PostBackEventTarget_IsExport');";
     2、客户端增加javascript函数AutoForm_Export。
        function AutoForm_Export(postBackEventTarget)
        {
           WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions(postBackEventTarget, "", true, "", "", false, true))    ;          
            return false;
        }
     3、在服务器端根据Request["__EVENTTARGET"] 来判断是否是导出事件。注意下面红色字体的注释   
        if (Page.Request["__EVENTTARGET"] != null)
        {
                        bool isInAsyncPostBack = false;
                        ScriptManager sm = ScriptManager.GetCurrent(Page);
                        if (sm != null)
                        {
                            isInAsyncPostBack = sm.IsInAsyncPostBack;
                        }
                        if (isInAsyncPostBack )
                        {
                            /*
                            一定要进行判断是否是使用Axaj进行异步请求，否则在点击导出按钮以后，再进行其它Ajax请求， Page.Request["__EVENTTARGET"] 会保留原来的值，这样就错误判断了。
                            */
                            _isExport = false

                        }
                        else
                        {
                            string postBackEventTarget = Page.Request["__EVENTTARGET"].ToString();
                            if (postBackEventTarget == "PostBackEventTarget_IsExport")
                            {
                                _isExport = true;
                            }
                            else
                                _isExport = false;
                        }
                    }
                    else
                        _isExport = false;

            }