比如这里是嵌套两个
<!DOCTYPE html>
<html>
<head>
<script src="/jquery/jquery-1.11.1.min.js">
</script>
</head>
<body>
<div id='outer'>
<div>
<div></div></div>
<div></div>
<div></div>
</div>
</body>
<script>
alert($('#outer div').length);
</script>
</html>
获得父div的div，.length