<script type="text/javascript" >
$(function(){
    var $comment = $('#comment');
    var $str  =  $('#str');
    var time;
    $comment.focus(function(){
        time = window.setInterval( substring,100 );
    });
    function substring() {
        var val = $comment.val();
        var length = val.length;
        if( $str.html() != (length) ){
            if(length==0){
                $("#wordcount")[0].firstChild.nodeValue = "您已输入0";
                $str.html(length);
            }else{
                $("#wordcount")[0].firstChild.nodeValue = "您已输入";
                $str.html(length);
            }
        }
    }
});
</script>

<p id="wordcount">
    您已输入<span id="str">0</span>字
</p>

via:http://wange.im/comment-count-in-wordpress-with-jquery.html