<div id="div1"></div>

#div1{width:100px;height:100px;background:red;position:absolute}

window.onload=function(){
  var oDiv=document.getElementById("div1");
  var x=0;
  var y=0;
  oDiv.onmousedown=function(ev){
    var oEvent=ev||event;
    //鼠标的横坐标减去div的offsetLeft
    x=oEvent.clientX-oDiv.offsetLeft;
    //鼠标的纵坐标减去div的offsetTop
    y=oEvent.clientY-oDiv.offsetTop; 
     
    document.onmousemove=function(ev){
      var oEvent=ev||event;   
      var left=oEvent.clientX-x;
      var right=oEvent.clientY-y;
      //判断左边是否过界
      if(left<0){
        left=0;
      }
      //判断右边是否过界
      else if(left>document.documentElement.clientWidth-oDiv.offsetWidth){
        left=document.documentElement.clientWidth-oDiv.offsetWidth;
      }
      //判断上边是否过界
      if(right<0){
        right=0;
      }
      //判断下边是否过界
      else if(right>document.documentElenment.clientHeight){
        right=document.documentElenment.clientHeight-oDiv.offsetHeight;
      }
      oDiv.style.left=left+"px";
      oDiv.style.top=right+"px";
    }   
    document.onmouseup=function(){
      //清空document的事件
      document.onmousemove=null;
      document.onmouseup=null;
    }
    //解决低版本火狐bug，干掉系统默认
    return false;
  }
}