直接把代码都Copy下来运行一下

<script language="javascript">
function getPosition() {
var top    = document.documentElement.scrollTop;
var left   = document.documentElement.scrollLeft;
var height = document.documentElement.clientHeight;
var width  = document.documentElement.clientWidth;
return {top:top,left:left,height:height,width:width};
}
function showPop(){
var width  = 300;  //弹出框的宽度
var height = 160;  //弹出框的高度
var obj    = document.getElementById("pop");
obj.style.display  = "block";
obj.style.position = "absolute";
obj.style.zindex   = "999";
obj.style.width    = width + "px";
obj.style.height   = height + "px";
var Position = getPosition();
leftadd = (Position.width-width)/2;
topadd  = (Position.height-height)/2;
obj.style.top  = (Position.top  + topadd)  + "px";
obj.style.left = (Position.left + leftadd) + "px";
window.onscroll = function (){
var Position   = getPosition();
obj.style.top  = (Position.top  + topadd)  +"px";
obj.style.left = (Position.left + leftadd) +"px";
};
}
function hidePop(){
document.getElementById("pop").style.display = "none";
}
</script>