SOURCE

$.fn.autoHeight = function(){
	
	function autoHeight(elem){
		elem.style.height = 'auto';
		elem.scrollTop = 0; //防抖动
		elem.style.height = elem.scrollHeight + 'px';
	}
	
	this.each(function(){
		autoHeight(this);
		$(this).on('keyup', function(){
			autoHeight(this);
		});
	});

}
	
	
$('textarea[autoHeight]').autoHeight();

<script id="jquery_183" type="text/javascript" class="library" src="http://sandbox.runjs.cn/js/sandbox/jquery/jquery-1.8.3.min.js"></script>
<div><textarea autoHeight="true">1</textarea></div>
<div><textarea autoHeight="true">2</textarea></div>
<div><textarea autoHeight="true">3</textarea></div>

body { background:#fff; }
textarea {width:300px; min-height:60px; overflow:hidden;}