1 <div class="cont">2     <div id="top"> </div>3     <div id="nav" class="nav"> </div>4 </div>5 <div style="height:1000px;"></div>

1  #top{width:100%;height:50px;border:1px solid red;}2  .nav{width:1080px;height:50px;background:green;margin:0px auto;}3  .scoll_nav{width:100%;position:fixed; top:10px;  z-index:10000;}

 1     $(document).ready(function(e) {                                    2         $(window).scroll(function() { 3           if($(window).scrollTop() >  $("#top").height()+10){ 4                  $("#nav").addClass("scoll_nav") 5             } 6           else{ 7                  $("#nav").removeClass(    "scoll_nav") 8             } 9        });                                       10     })