• css

       <style type="text/css">
             body{height: 2500px; margin: 0; padding: 0;}
            .banner{height: 250px; width: 100%; background: #e5e5e5;}
            .bignav{width: 100%; background: #000;}
            .nav{ background:#000; width: 1200px; margin: 0 auto; height: 45px;}
            .nav a{display: block; width: 200px;float: left; color: #fff; text-decoration: none; text-align: center; line-height: 45px;}
        </style>

• html

      <div class="banner">
      </div>
      <div class="bignav" id="bignav">
          <div class="nav">
              <a href="#">首页</a>
              <a href="#">导航1</a>
              <a href="#">导航1</a>
              <a href="#">导航1</a>
          </div>
      </div>
      <p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Aperiam quis, ducimus reiciendis fugit provident reprehenderit quam, soluta quisquam, tenetur voluptatum quibusdam eum. Necessitatibus molestias natus dolores tempora reiciendis eum dolorum.</p>
      <p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Aperiam quis, ducimus reiciendis fugit provident reprehenderit quam, soluta quisquam, tenetur voluptatum quibusdam eum. Necessitatibus molestias natus dolores tempora reiciendis eum dolorum.</p>
      <p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Aperiam quis, ducimus reiciendis fugit provident reprehenderit quam, soluta quisquam, tenetur voluptatum quibusdam eum. Necessitatibus molestias natus dolores tempora reiciendis eum dolorum.</p>
      <p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Aperiam quis, ducimus reiciendis fugit provident reprehenderit quam, soluta quisquam, tenetur voluptatum quibusdam eum. Necessitatibus molestias natus dolores tempora reiciendis eum dolorum.</p>

• js

            window.onscroll=function(){
                var topScroll =document.documentElement.scrollTop;
                console.log(111111,topScroll)
                var bignav  = document.getElementById("bignav");//获取到导航栏id
                if(topScroll > 250){  //当滚动距离大于250px时执行下面的东西
                    bignav.style.position = 'fixed';
                    bignav.style.top = '0';
                    bignav.style.zIndex = '9999';
                }else{//当滚动距离小于250的时候执行下面的内容，也就是让导航栏恢复原状
                    bignav.style.position = 'static';
                }
            }