SOURCE

window.onload = function() {
  var btn = document.getElementById('btn');
  var png = document.getElementById('png');
  var sudu = 10;

  btn.onclick = function() {
    setInterval(function() {
      var laoleft = png.style.left;
      var xinleft = parseInt(laoleft) + sudu;
      png.style.left = xinleft + 'px';
      if (xinleft == 600 || xinleft == 50) {
        sudu = sudu * -1;
      }
    },
    20)
  }
}

<input id="btn" type="button" value="点击来回运动">
<div id="png" style="left: 50px;">
</div>

#png {
  width: 200px;
  height: 200px;
  background-color: blue;
  position: absolute;
  top: 80px;
}

#btn {
  width: 100px;
  height: 50px;
  background-color: red;
  color: #fff;
  border: none;
  font-size: 14px;
}