<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(){

?//獲取要定位元素距離瀏覽器頂部的距離

?var navH = $(".nav").offset().top;

?//滾動條事件

?$(window).scroll(function(){

?//獲取滾動條的滑動距離

?var scroH = $(this).scrollTop();

?//滾動條的滑動距離大于等于定位元素距離瀏覽器頂部的距離，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"});

?}

?})

?})