<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
body{
height:2000px;
}
#box{
width: 300px;
height:300px;
position: absolute;
background-color: #8c8c8c;
}
</style>
<script>
window.onload = function(){
function pop (){
var scrollTop = document.body.scrollTop;//滚动条到顶部
var oBox = document.getElementById('box');
var oClose = document.getElementById('close');
oBox.style.display = 'block';
oBox.style.top=document.documentElement.clientHeight/2-oBox.offsetHeight/2+scrollTop+'px';
oBox.style.left=document.documentElement.clientWidth/2-oBox.offsetWidth/2+'px';
}pop();
}
</script>
</head>
<body>
<div id="box"></div>
</body>
</html>