<form id="myFormId" name="myForm" action="" method="post">
 <input type="hidden" id="inParam" name="inParam" />
 <input type="hidden" id="tstype" name="tstype" />
 <input type="hidden" id="zd" name="zd" />
</form>

document.myForm.action = "userinfo.shtml";  

//需要注意的是form标签一定要有name属性，光有id没有name不起作用

document.myForm.attributes["action"].value = "XXXXXXXX";

document.all("myFormId").setAttribute("action", "欲赋的action的值");

$("#myFormId").attr("action", "userinfo.shtml");