SOURCE

$(window).scroll( function() { 
  if($(document).scrollTop()>$("#part3box").offset().top){
    $("#part3").addClass("part3");
  }else{
    $("#part3").removeClass("part3");
  }
})

<div id="bigwrap">
  <div id="part0"></div>
  <div id="part1"></div>
  <div id="part2"></div>
  <div id="part3box"><div id="part3">吸顶内容</div></div>
  <div id="part4"></div>
  <div id="part5"></div>
</div>

*{margin: 0;padding: 0;}
#bigwrap{
  width: 100%;
  overflow: hidden;
}
#part0{
  width: 100%;
  height: 80px;
  background-color: lightgoldenrodyellow;
}
#part1{
  width: 100%;
  height: 500px;
  background-color: #F1F1F1;
}
#part2{
  width: 100%;
  height: 400px;
  background-color: lightblue;
}
#part3box{
  width: 100%;
  height: 100px;
}
#part3{
  width: 100%;
  height: 100px;
  background-color: palegreen;
  font-size: 30px;
  color: #F3F3F3;
  line-height: 100px;
  text-align: center;
  font-weight: bold;
}
.part3{
  position:fixed;
  left:0;
  top:0;
}
#part4{
  width: 100%;
  height: 350px;
  background-color:#F1F1F1;
}
#part5{
  width: 100%;
  height: 450px;
  background-color: paleturquoise;
}