<style>
.main{min-height: 700px;}
</style>
<div class="header"></div>
<div class="main"></div>
<div class="footer"></div>
还有一种方法是给body相对定位，给这个div 绝对定位到底部
<body>
<style>
body{position: relative;height: 1000px;}
.footer{position: absolute;bottom: 0;left: 0;height: 20px;width: 100%;background-color: yellow;}
</style>
<div class="header"></div>
<div class="main"></div>
<div class="footer"></div>
</body>