SOURCE

$(function () {
    var position = $("#select_btn").offset();
    $("#div1").offset({
        top: position.top + 22,
        left: position.left
    });
    $("#select_btn").click(function () {
        $("#NG").toggleClass("open");
        document.onclick = function () {
            $("#NG").toggleClass("close");
        }
    });
    $("#div1 input[name=ng]").click(function () {
        var arr = new Array();
        $("input[name=ng]:checked").each(function (key, obj) {
            arr[key] = $(obj).val();
        });
        $("#select_btn").val(arr.join(","));
    })

})

//阻止冒泡函数
function stopBubble(e) {
    if (e && e.stopPropagation) {
        e.stopPropagation();  //w3c
    } else {
        window.event.cancelBubble = true; //IE
    }
}

<div id="div">
	<div align="center" id="div2">
		<input type="text" readonly="readonly" id="select_btn"/>
    </div>
    <div id="div1">
        <ul class="close" id="NG" >
            <li><input type="checkbox" name="ng" value=1 />1</li>
            <li><input type="checkbox" name="ng" value=2 />2</li>
            <li><input type="checkbox" name="ng" value=3 />3</li>
        </ul>
    </div>
</div>

#div {
    margin-bottom: 10px;
    position: relative;
}

#div1 {
    width: 153px;
    padding-top: 0px;
    padding-left: 0px;
    position: absolute;
}

#div1 ul {
    margin-top: 0px;
    padding-left: 0px;
    background-color: #ccc;
    list-style: none;
}

#div1 ul li {
    padding-left: 0px;
}

#div1 ul li input {
    margin-left: 15px;
}

.close {
    display: none;
}

.open {
    display: block;
}