如果我没有数错的话，你在代码的最后少了一个"}"反括号，加上即可，除此之外 This.style.height = this.offsetHeight + 16 +"px";中的this应为oSubNav也就是var中的This，下同。 附代码：

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
            var oSubNav=this.getElementsByTagName('ul')[0];
            if(oSubNav){
            var This=oSubNav;
            clearInterval(This.time);
            This.time=setInterval(function(){
                    This.style.height=This.offsetHeight+16+"px";
                    if(This.offsetHeight>=120){
                    clearInterval(This.time);}
                },30)
             }
          }
    
    
    
    
    
    
        //鼠标离开菜单，二级菜单动画收缩起来。		
			
            aLi[i].onmouseout=function(){
            var oSubNav=this.getElementsByTagName('ul')[0];
             if(oSubNav) {
                 var This = oSubNav;
                 clearInterval(This.time);
                 This.time = setInterval(function() {
                    This.style.height = This.offsetHeight - 16 +"px";
                    if (This.offsetHeight <= 0) {
                        clearInterval(This.time);
                    }
                 },30)
            
            
            
            
            
            
		    }
		
         }	
	}
}