对于当前标签元素来说的

<script>
window.onload=function(){
    var aLi=document.getElementsByTagName('li');
 for(var i=0; i<aLi.length; i++){
  aLi[i].onmouseover=function(){
            //鼠标经过一级菜单，二级菜单动画下拉显示出来
         self = this;
            var subNav = self.getElementsByTagName("ul")[0];
            if(subNav){
                clearInterval(subNav.time);
                subNav.time = setInterval(function(){//原先写成self.time就有错误了
                    subNav.style.height = subNav.offsetHeight + 16 +"px";
                    if(subNav.offsetHeight >= 120){
                        clearInterval(subNav.time);
                    }
                },30);    
            }
  }
        //鼠标离开菜单，二级菜单动画收缩起来。  
  aLi[i].onmouseout=function(){
   self = this;
            var subNav = self.getElementsByTagName("ul")[0];
            if(subNav){
                clearInterval(subNav.time);
                subNav.time = setInterval(function(){
                    subNav.style.height = subNav.offsetHeight - 16 +"px";
                    if(subNav.offsetHeight == 0){
                        clearInterval(subNav.time);
                    }
                },30);    
            }
                   
  }  
  
 }
}
</script>

用self保存aLi[i]的this对象?如果没这句，你后面的写法没有区别，都是给对象属性赋值。而this在不同环境下意义不一下，具体情况具体分析，贴出你写的代码.