<script type="text/javascript">
	var scrollFunction=function(e){
	    e=e || window.event;  
	    var _h = document.documentElement.clientHeight;
	    var _s = document.documentElement.scrollTop||document.body.scrollTop;
	    var _m = _s + (_h - 500) * 0.5;
	    if (_m < 0) {
	        _m = 0;
	    }
	    document.getElementById("divid").style.top=_m+"px";
	};  
	//FireFox
	if(document.addEventListener){
		document.addEventListener("DOMMouseScroll" ,scrollFunction, false);
	}
	//IE、Opera、Chrome  
	window.onscroll=document.onscroll=scrollFunction;
</script>
 

500是div容器的高，可用document.getElementById("divid").style.height獲取，不過得到的值是"500px"