1.居中弹出层 
#prompt_center_div{
		border:1px solid gray;
		position:fixed;
		background: #f2f2f2;
		top: 50%;
		left: 50%;
		padding:5px;
		/* opacity:0.1;
		-moz-opacity:0.1;
		filter:alpha(opacity=10); */
		}
<!-- 弹出层 -->
		<div id="prompt_center_div">
			姓名:小小明<br/>
			电话：<input type="text" value="" name="" id=""/>
		</div>

2.获得屏幕中心对象获得屏幕中心对象
function getPosition() { 
	var top,left,height,width,h,w; 
	
	if(document.compatMode && document.compatMode!="BackCompat"){
		top = document.documentElement.scrollTop; 
		left = document.documentElement.scrollLeft; 
		height = document.documentElement.clientHeight; 
		width = document.documentElement.clientWidth; 
		h = document.documentElement.scrollHeight;
		w = document.documentElement.scrollWidth;
	}else{
		top = document.body.scrollTop; 
		left = document.body.scrollLeft; 
		height = document.body.clientHeight; 
		width = document.body.clientWidth; 
		h = document.body.scrollHeight;
		w = document.body.scrollWidth;
	}
	if(h<height){
		h = height;
		w = width;
	}

//	alert(height + '---------' + h + '---------' + width + '-----------' + w);
	
	return {top:top,left:left,height:height,width:width,h:h,w:w,sw:window.screen.width,sh:window.screen.height}; 
}

function getCenter() {
	var pos = getPosition();
	return {left:pos.left+pos.width/2,top:pos.top+pos.height/2,height:pos.height,width:pos.width,h:pos.h,w:pos.w,sw:pos.sw,sh:pos.sh};
}