导航跟随滚动条置顶移动示例代码

#topmenu{position:absolute;top:100px;background:#4b4a4a;width:100%;margin:0 auto;clear:both;overflow:hidden;z-index:998;left:0} 

<script type="text/javascript"> 
jQuery(document).ready(function() { 
var topmenu = jQuery("#topmenu"); 
var topmenu_top = topmenu.offset().top; 
reset_topmenu_top(topmenu, topmenu_top); 
jQuery(window).scroll(function() { 
reset_topmenu_top(topmenu, topmenu_top); 
}); 
}); 
function reset_topmenu_top(topmenu, topmenu_top) { 
var document_scroll_top = jQuery(document).scrollTop(); 
if (document_scroll_top > topmenu_top) { 
topmenu.css('top', document_scroll_top); 
} 
if (document_scroll_top <= topmenu_top) { 
topmenu.css('top', topmenu_top); 
} 
} 
</script>