<div class="wrap">
 <div class="cover">

 </div>
</div>

* {
  margin: 0;
  padding: 0;
 }
 
 .wrap {
  width: 500px;
  height: 500px;
  border: 1px solid deeppink;
  position: relative;
  margin: 50px auto;
 }
 
 .cover {
  width: 150px;
  height: 150px;
  background: rgba(200, 7, 99, 0.5);
  display: none;
  position: absolute;
  left: 0;
  top: 0;
  cursor: move;
 }

<script>
 var wrap = document.querySelector(".wrap");
 var cover = document.querySelector(".cover");
 wrap.onmouseover = function() {
 cover.style.display = "block";
 wrap.onmousemove = function(e) {
  var e = e || window.event;
  var x1 = e.clientX;
  var y1 = e.clientY;
//这里获取到的e.clientX和e.clientY,可以看情况和需求改为e.pageX和e.pageY  
  var halfWidth = cover.clientWidth / 2;
  var halfHeight = cover.clientHeight / 2;
  var wrapLeft = wrap.offsetLeft;
  var wrapTop = wrap.offsetTop;
  var l = x1 - wrapLeft - halfWidth;
  var t = y1 - wrapTop - halfHeight;

  if (l <= 0) {
  l = 0
  }
  if (l >= wrap.clientWidth - cover.clientWidth) {
  l = wrap.clientWidth - cover.clientWidth
  }
  if (t <= 0) {
  t = 0
  }
  if (t >= wrap.clientHeight - cover.clientHeight) {
  t = wrap.clientHeight - cover.clientHeight
  }
  cover.style.left = l + "px";
  cover.style.top = t + "px"
 }
 }
 wrap.onmouseout = function() {
 cover.style.display = "none";
 }
</script>

var halfWidth = cover.clientWidth / 2;
var halfHeight = cover.clientHeight / 2;
var wrapLeft = wrap.offsetLeft;
var wrapTop = wrap.offsetTop;
var l = x1 - wrapLeft - halfWidth;
var t = y1 - wrapTop - halfHeight;
//限制范围
  if (l <= 0) {
    l = 0
   }
   if (l >= wrap.clientWidth - cover.clientWidth) {
    l = wrap.clientWidth - cover.clientWidth
   }
   if (t <= 0) {
    t = 0
   }
   if (t >= wrap.clientHeight - cover.clientHeight) {
    t = wrap.clientHeight - cover.clientHeight
   }