background-attachment 该属性来定义背景图片随滚动轴的移动方式

取值: scroll | fixed | inherit

scroll: 随着页面的滚动轴背景图片将移动

fixed: 随着页面的滚动轴背景图片不会移动

inherit: 继承

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css" media="all">
			body 
			{
				background-image:url('img.jpg');
				background-size: 100% 100%;
				background-attachment:fixed;/*定义背景图片随滚动轴的移动方式*/				 
				background-repeat:no-repeat; 
				background-position:top center;		
			
			}
			.box{
				width: 1000px;
				margin:0 auto;
				border:2px solid red;
				height: 5000px;
				line-height: 80px;
				font-size: 30px;
				color:blue ;
			}
		</style>
	</head>	
	<body>
		 	<div class="box"></div>		 
	</body>
</html>