<html>
<head>
<title>test</title>
</head>
<body>
<div style="position: fixed; height: 200px; background: red; width: 100%; display: none" id="topbar"></div>
<div style="height: 3000px;">content</div>
</body>
<script type="text/javascript">
window.onscroll = function () {
var top = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop;
var node = document.getElementById('topbar');
if (top > 500) {//500就是滚动条滚动到的位置，大于500才显示
node.style.display = 'block';
} else {
node.style.display = 'none';
}
}
</script>
</html>
