<script type="text/javascript" src="http://code.jquery.com/jquery-latest.js"></script>
<script>
$(function() {
    var elm = $('#pordAttr');
    var startPos = $(elm).offset().top;
    $.event.add(window, "scroll", function() {
        var p = $(window).scrollTop();
        $(elm).css('position',((p) > startPos) ? 'fixed' : 'static');
        $(elm).css('top',((p) > startPos) ? '0px' : '');
    });
});
</script>

•$(function(){

•//获取要定位元素距离浏览器顶部的距离

•var navH = $(".nav").offset().top;

•//滚动条事件

•$(window).scroll(function(){

•//获取滚动条的滑动距离

•var scroH = $(this).scrollTop();

•//滚动条的滑动距离大于等于定位元素距离浏览器顶部的距离，就固定，反之就不固定

•if(scroH>=navH){

•$(".nav").css({"position":"fixed","top":0});

•}else if(scroH<navH){

•$(".nav").css({"position":"static"});

•}

•})

•})