objFloatBox.style.left 是floatBox相对于谁来讲的距离呢？

var left=_event.clientX-objDemo.offsetLeft-objSmallBox.offsetLeft-objFloatBox.offsetWidth/2;

var top=_event.clientY-objDemo.offsetTop-objSmallBox.offsetTop-objFloatBox.offsetHeight/2;

objFloatBox.style.left=left + "px";

objFloatBox.style.top=top + "px";