<head>
    <title>ALong7yrone</title>
    <style type="text/css">
        .mydiv
        {
            height: 31px;
            margin: 20px 0;
            background-color: #fff;
            border-bottom: 1px solid #ccc;
        }
        .mydiv span
        {
            display: inline-block;
            height: 20px;
            line-height: 20px;
            border: 1px solid #ccc;
            padding: 5px;
            margin-right: 10px;
            border-bottom: 0;
            background-color: #fff;
        }
    </style>
    <script type="text/javascript">
        var mt = 0;
        window.onload = function () {
            var mydiv = document.getElementById("mydiv");
            var mt = mydiv.offsetTop;
            window.onscroll = function () {
                var t = document.documentElement.scrollTop || document.body.scrollTop;
                if (t > mt) {
                    mydiv.style.position = "fixed";
                    mydiv.style.margin = "0";
                    mydiv.style.top = "0";
                }
                else {
                    mydiv.style.margin = "20px 0";
                    mydiv.style.position = "static";
                }
            }
        }
        
    </script>
</head>
<body>
    <div style="height: 400px; border: 1px solid #000;">
    </div>
    <div class="mydiv" id="mydiv">
        <span style="position: relative; top: 1px;">商品详情</span><span>评价信息</span>
　　</div>
</body>

在此后添加N多行

<div>
        效果演示
        <br />
    　　。。。。。。。。。。N多行
</div>

不过有些简陋。