js代码如下：

if ($('#follow-us')){
    var followUsPosition = $('#follow-us').offset().top;
    window.onscroll = function () {
        var nowPosition =  document.documentElement.scrollTop;
        if (nowPosition - followUsPosition > 0 ) {
            setTimeout(function () {
                $('#follow-us').attr('class','guanzhu gd');
            },150);
        }else {
            $('#follow-us').attr('class','guanzhu');
        }
    };
}
});

Html:   

  <div class="guanzhu" id="follow-us">
      <h2 class="hometitle">关注我们 么么哒！</h2>
      <ul>
        <li class="sina"><a href="/" target="_blank"><span>新浪微博</span>杨青博客</a></li>
        <li class="tencent"><a href="/" target="_blank"><span>腾讯微博</span>杨青博客</a></li>
        <li class="qq"><a href="/" target="_blank"><span>QQ号</span>476847113</a></li>
        <li class="email"><a href="/" target="_blank"><span>邮箱帐号</span>dancesmiling@qq.com</a></li>
        <li class="wxgzh"><a href="/" target="_blank"><span>微信号</span>yangqq_1987</a></li>
        <li class="wx"><img src="images/wx.jpg"></li>
      </ul>
    </div>

/*gd*/

.gd { width: 300px; top: 100px; z-index: 999; position: fixed; _position: absolute; _top:expression(documentElement.scrollTop + 0 + "px");
_margin-top: 100px; }