Ajax post提交方式要加request.setRequestHeader

 var url = "Back.aspx";
    request.open("post",url,true);

//Post时要加头的
    request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");   
    request.onreadystatechange = updatePage;
    var str = "par=cccccccccccccccccccccc";
    request.send(str);