SOURCE

function menuFixed(id) {
				var obj = document.getElementById(id);
				var _getHeight = obj.offsetTop;
 
				window.onscroll = function() {
					changePos(id, _getHeight);
				}
			}
 
			function changePos(id, height) {
				var obj = document.getElementById(id);
				var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
				if(scrollTop < height) {
					obj.style.position = 'relative';
				} else {
					obj.style.position = 'fixed';
				}
			}
		</script>
		<script type="text/javascript">
			window.onload = function() {
				menuFixed('nav_keleyi_com');
			}

<div class="wrapper">
			<div class="header">haha</div>
			<div id="nav_keleyi_com">
				<a href="http://keleyi.com/menu/net/" target="_blank">.NET</a>
				<a href="http://keleyi.com/menu/javascript/" target="_blank">Javascript</a>
				<a href="http://keleyi.com/menu/jquery/" target="_blank">jQuery</a>
				<a href="http://keleyi.com/menu/csharp/" target="_blank">C#</a>
				<a href="http://keleyi.com/menu/other/" target="_blank">其他</a>
				<a href="http://keleyi.com/" target="_blank">首页</a>
				<a href="/menu/aspnet/" target="_blank">ASP.NET</a>
				<a href="/menu/mvc/" target="_blank">MVC</a>
			</div>
			<div>hhehehehh</div>
		</div>

.wrapper {
				width: 1000px;
				height: 2000px;
				margin-left: auto;
				margin-right: auto;
			}
			
			.header {
				height: 150px;
			}
			
			#nav_keleyi_com {
				padding: 10px;
				position: relative;
				top: 0;
				background: #125430;
				width: 1000px;
			}
			
			a {
				display: inline-block;
				margin: 0 10px;
				*display: inline;
				zoom: 1;
				color: white;
			}