<span style="display:inline-block;position:relative" class="combox_border">
 <input type="text" class="combox_input"/><font class="ficomoon icon-angle-bottom combox_button" style="display:inline-block"></font>
 <ul style="position:absolute;top:29px;left:-1px" class="combox_select">
 <li><a href="javascript:void(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >选项一</a></li>
 <li><a href="javascript:void(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >选项二</a></li>
 <li><a href="javascript:void(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >选项三</a></li>
 <li><a href="javascript:void(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >选项四</a></li>
 </ul>
</span>

@font-face {
 font-family: 'icomoon';
 src:url('fonts/icomoon.eot?-fl11l');
 src:url('fonts/icomoon.eot?#iefix-fl11l') format('embedded-opentype'),
 url('fonts/icomoon.woff?-fl11l') format('woff'),
 url('fonts/icomoon.ttf?-fl11l') format('truetype'),
 url('fonts/icomoon.svg?-fl11l#icomoon') format('svg');
 font-weight: normal;
 font-style: normal;
}
.ficomoon{font-family:'icomoon';}
.icon-angle-top:before {content: "\f102"}.icon-angle-bottom:before {content: "\f103"}
/*下面的可根据自己的实际情况做修改*/
.combox_border{border:1px solid #c2c2c2;height:28px;width:245px}
.combox_input{border:0;line-height:25px;height:25px;padding-left: 5px;width:85%;vertical-align: middle;}
.combox_button{width:12%;text-align:center;vertical-align: middle;cursor:pointer;border-left:1px solid #c2c2c2}
.combox_select{border:1px solid #c2c2c2;border-top:0;width:100%}
.combox_select li{overflow:hidden;height:30px;line-height:30px;cursor:pointer;}
.combox_select a {display: block;line-height: 28px;padding: 0 8px;text-decoration: none;color: #666;}
.combox_select a:hover {text-decoration: none;background:#f5f5f5}

(function($){
 $.fn.combox = function(options) {
 
 };
})(jQuery);

var defaults = { 
 borderCss: "combox_border", 
 inputCss: "combox_input", 
 buttonCss: "combox_button", 
 selectCss: "combox_select",
 datas:[]
};
var options = $.extend(defaults, options);

this.each(function() {
var _this = $(this);
_this = _initBorder(_this);//初始化外框CSS IE6中需要有返回值
_this = _initInput(_this);//初始化输入框
_initSelect(_this);//初始化下拉列表
});

　　　　　　 function _initBorder($border) {//初始化外框CSS
 $border.css({'display':'inline-block', 'position':'relative'}).addClass(options.borderCss);
 return $border;
 }
 
 function _initInput($border){//初始化输入框
 $border.append('<input type="text" class="'+options.inputCss+'"/>');
 $border.append('<font class="ficomoon icon-angle-bottom '+options.buttonCss+'" style="display:inline-block"></font>');
 //绑定下拉特效
 $border.delegate('font', 'click', function() {
 var $ul = $border.children('ul');
 if($ul.css('display') == 'none') {
 $ul.slideDown('fast');
 $(this).removeClass('icon-angle-bottom').addClass('icon-angle-top');
 }else {
 $ul.slideUp('fast');
 $(this).removeClass('icon-angle-top').addClass('icon-angle-bottom');
 } 
 });
 return $border;//IE6需要返回值
 }
 
 function _initSelect($border) {//初始化下拉列表
 $border.append('<ul style="position:absolute;left:-1px;display:none" class="'+options.selectCss+'">');
 var $ul = $border.children('ul');
 $ul.css('top',$border.height()+1);
 var length = options.datas.length;
 for(var i=0; i<length ;i++)
 $ul.append('<li><a href="javascript:void(0)" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" rel="external nofollow" >'+options.datas[i]+'</a></li>');
 $ul.delegate('li', 'click', function() {
 $border.children(':text').val($(this).text());
 $ul.hide();
 $border.children('font').removeClass('icon-angle-top').addClass('icon-angle-bottom');//确定的时候要将下拉的icon改变
 });
 return $border;
 }

<script type="text/javascript">
$(document).ready(function() {
 $('#combox').combox({datas:['选项一','选项二','选项三']});
})
</script>
</head>
<body>
　　<span id="combox"></span>
</body>
</html>