<div id="Jnav">
 <ul class="nav">
 <li><a href="http://caibaojian.com/">WEB前端开发</a></li>
 <li><a href="#">前端开发博客</a></li>
 <li><a href="#">前端开发</a></li>
 <li><a href="#">前端开发</a></li>
 </ul>
</div>

var $nav = $('#Jnav'), navTop = $nav.offset().top, navH = $nav.outerHeight(),winTop_1=0,winWidth=$(window).width(), holder=jQuery('<div>');
$(window).on('scroll',function(){
 var winTop_2 = $(window).scrollTop();
 holder.css('height',navH);
 //开始浮动，不过不显示
 if(winTop_2>navTop && winWidth>980){
 holder.show().insertBefore($nav);
 $nav.addClass('fixed-nav');
 }else{
 holder.hide();
 $nav.removeClass('fixed-nav');
 }
 //判断鼠标向上滚动，显示出来
 if(winTop_2>winTop_1 && winWidth>980){
 $nav.removeClass('fixed-nav-appear');
 }else if(winTop_2<winTop_1){
 $nav.addClass('fixed-nav-appear');
 }
 winTop_1 = $(window).scrollTop();
})

.nav{width:980px; margin:0 auto;}
.nav li{display:inline-block; *display:inline; *zoom:1; margin:0 10px;}
.nav li a{display:block; padding:5px 10px;}
.fixed-nav{
 position: fixed;
 width:100%;
 top:-40px; 
 -webkit-transition: top .5s;
  -moz-transition: top .5s;
  -o-transition: top .5s;
  transition: top .5s;
  -webkit-box-shadow: 0 2px 2px rgba(0,0,0,.1);
  -moz-box-shadow: 0 2px 2px rgba(0,0,0,.1);
  box-shadow: 0 2px 2px rgba(0,0,0,.1);
}
.fixed-nav-appear{top:0;}