<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
*{margin: 0;padding: 0;}
html,body{height: 3000px}
#box{
100%;
height: 100px;
background: red;
position: fixed;
top: 300px;
text-align: center;
line-height: 100px;
}
</style>
</head>
<body>
<div id="box">横条</div>
</body>
</html>
<script type="text/javascript">
var b=document.getElementById("box");
window.onscroll=function(){
var sTop=document.documentElement.scrollTop||document.body.scrollTop;
if(sTop>300){
box.style.top=0;
}else{
box.style.top=parseInt(300-sTop)+"px";
}
}
</script>