* {
 padding: 0;
 margin: 0;
}
#box1 {
 width: 500px;
 height: 500px;
 background: #999;
 position: relative;
 left: 100px;
 top: 100px;
}
#box {
 width: 100px;
 height: 100px;
 background: #334;
 position: absolute;
 cursor: move;
}

<div id="box1">
<div id="box"></div>
</div>

(function () {
 var dragging = false
 var boxX, boxY, mouseX, mouseY, offsetX, offsetY
 var box = document.getElementById('box')
 var box1 = document.getElementById('box1')

 // 鼠標(biāo)按下的動作
 box.onmousedown = down
 // 鼠標(biāo)的移動動作
 document.onmousemove = move
 // 釋放鼠標(biāo)的動作
 document.onmouseup = up

 // 鼠標(biāo)按下后的函數(shù),e為事件對象
 function down(e) {
  dragging = true

  // 獲取元素所在的坐標(biāo)
  boxX = box.offsetLeft
  boxY = box.offsetTop

  // 獲取鼠標(biāo)所在的坐標(biāo)
  mouseX = parseInt(getMouseXY(e).x)
  mouseY = parseInt(getMouseXY(e).y)

  // 鼠標(biāo)相對元素左和上邊緣的坐標(biāo)
  offsetX = mouseX - boxX
  offsetY = mouseY - boxY
 }

 // 鼠標(biāo)移動調(diào)用的函數(shù)
 function move(e){
  if (dragging) {
   // 獲取移動后的元素的坐標(biāo)
   var x = getMouseXY(e).x - offsetX
   var y = getMouseXY(e).y - offsetY

   // 計(jì)算可移動位置的大小， 保證元素不會超過可移動范圍
   // 此處就是父元素的寬度減去子元素寬度
   var width = box1.clientWidth - box.offsetWidth
   var height = box1.clientHeight - box.offsetHeight

   // min方法保證不會超過右邊界，max保證不會超過左邊界
   x = Math.min(Math.max(0, x), width)
   y = Math.min(Math.max(0, y), height)

   // 給元素及時(shí)定位
   box.style.left = x + 'px'
   box.style.top = y + 'px'
  }
 }

 // 釋放鼠標(biāo)的函數(shù)
 function up(e){
  dragging = false
 }

 // 函數(shù)用于獲取鼠標(biāo)的位置
 function getMouseXY(e){
  var x = 0, y = 0
  e = e || window.event

  if (e.pageX) {
   x = e.pageX
   y = e.pageY
  } else {
   x = e.clientX + document.body.scrollLeft - document.body.clientLeft
   y = e.clientY + document.body.scrollTop - document.body.clientTop
  }
  return {
   x: x,
   y: y
  }
 }
})()

// 此處就是父元素的寬度減去子元素寬度
var width = box1.clientWidth - box.offsetWidth
var height = box1.clientHeight - box.offsetHeight