也就是说让一个div限定在另一个div的区域里面（类似于css background-image中大尺寸图片定义在小尺寸的div中，图像只显示其在小尺寸div中的部分－－－但是这里不用background-image，而是换成一个div，就不知道怎么实现了 ╮(￣ ￣)╭）
比如这个随便写的这个网页，想让 .phone:hover 上移的div只显示其在手机屏幕中的部分
你自己看下，先考虑下布局
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>关注ReSur的微信</title>
<style type="text/css">
body {margin: 0;background-color: #FAFAFA;}
.block {position: relative;margin-top: 275px;width: 100%;height: 195px;background: #00C389;}
.draw {position: relative;margin: 0 auto;height: 391px;height: 579px;background: url('http://1992s.com/images/wechat-bg.png') no-repeat;}
.draw_box{margin: 134px 0 0 409px; background:#000; width:219px; height:391px; float:left; overflow:hidden; position:relative;}
.phone {position: absolute;top: 137px;left:0;width: 219px;height: 391px;text-align: center;background: white;transition: top 1s;-webkit-transition: top 1s;-moz-transition: top 1s;-o-transition: top 1s;}
.phone:hover {top: 0px;}
.qrcode {position: relative;top: 87px;margin: 0 auto;width: 165px;height: 165px;background: url('http://1992s.com/images/wechat.png') no-repeat;}
.text {position: relative;top: 140px;font: 12px "Lucida Grande","Lucida Sans Unicode",Helvetica,Arial,Verdana,sans-serif,"Microsoft YaHei","微软雅黑";}
</style>
<link rel="shortcut icon" href="/favicon.ico" type="image/x-icon" />
<script type="text/javascript" src="http://1992s.com/wp-content/themes/portfolio-press-child/js/info.js"></script>
</head>
<body>
<div class="block">
<div class="draw">
<div class="draw_box">
<div class="phone">
<div class="qrcode"></div>
<div class="text">
<p>或在“查找公众号”<br />中搜索“ReSur”</p>
</div>
</div>
</div>
</div>
</div>
</body>
</html>