<script src="https://cdn.staticfile.org/jquery/1.10.2/jquery.min.js"> </script>
<script> function sub(){ alert($("#aaa").val()); } </script>
</head>
<body>
<select id="aaa">
  <option>Volvo</option>
  <option>Saab</option>
  <option>Mercedes</option>
  <option>Audi</option>
</select>
	<button id="aa" onclick="sub()">点击</button>
</body>
</html>