function scrollPosition(id,p_top) {
 
   /*获取某个元素的相对偏移，此元素必须是可见的，返回值是top 和left 单位是像素 移动到固定元素上尽可能使用padding
    *
    *
    *
    * */
   var offset= $(id).offset();
   console.log(offset);
 
   $('body,html').animate({
     scrollTop:offset.top+p_top
   })
 };
 
 $("#pingtai").click(function () {
   scrollPosition('#pingtaijieshao','-800')
 })
 $("#jishu").click(function () {
   scrollPosition('#jishutedian','-800')
 })

<ul class="menhu_topMenuClass pingtai_TopMenuClass">
      <li>
        <a href="javascript:void(0)" rel="external nofollow" rel="external nofollow" class="on" id="pingtai">平台介绍</a>
      </li>
      <li>
        <a href="javascript:void(0)" rel="external nofollow" rel="external nofollow" class="end" id="jishu">技术特点</a>
      </li>
 
    </ul>