js方法：

function checkLength(which) {
        var maxChars = 200;
        if (which.value.length > maxChars)
        which.value = which.value.substring(0,maxChars);
        var curr = maxChars - which.value.length;
        document.getElementById("lblRemarksNote_Cur").innerHTML = "最多输入200个字，您已输入" + which.value.length +"个字，(还可以输入" + curr +"个字）";
        }

html页面元素：

 <textarea ID="txtRemarks"  TextMode="MultiLine" onkeyup="checkLength(this);">
                        </textarea ><span id="lblRemarksNote_Cur"></span>