Edit in JSFiddle

$(function(){
	var lock = function(){
		$('#desktop').hide();
		$('body').append('<div id="lock"><input type="button" id="unlockbtn" value="关闭锁屏"></div>');
		lockFunc = setInterval(function(){
			//检查是否有恶意修改源程序绕过锁屏界面
			var iswarning = false;
			if($('#desktop').is(':visible')){
				iswarning = true;
			}
			if($('#lock').length == 0){
				iswarning = true;
			}
			if($('#lock').is(':hidden')){
				iswarning = true;
			}
			//如果有则重新载入锁屏
			if(iswarning){
				clearInterval(lockFunc);
				$('#lock').remove();
				lock();
				alert('别干坏事哦！');
			}
		}, 500);
	};
	var unlock = function(){
		clearInterval(lockFunc);
		$('#lock').remove();
		$('#desktop').show();
	};
	$('#lockbtn').click(function(){
		lock();
	});
	$('body').on('click', '#unlockbtn', function(){
		unlock();
	});
});

<div id="desktop">
	<input type="button" id="lockbtn" value="开启锁屏">
</div>

body{margin:0;padding:0}
#desktop{position:absolute;width:100%;height:100%;background:#F00}
#lockbtn{position:absolute;left:50%;top:50%;margin-left:-50px;margin-top:-25px;width:100px;height:50px;font-size:18px}
#lock{position:absolute;width:100%;height:100%;background:#00F}
#unlockbtn{position:absolute;right:10px;bottom:10px;width:100px;height:50px;font-size:18px}