<script type="text/javascript">
    function getPosition(obj) {
        var result = 0;
        if (obj.selectionStart) { //非IE浏览器
            result = obj.selectionStart
        }
        else
        { //IE
            var rng;
            if (obj.tagName == "TEXTAREA") { //如果是文本域
                rng = event.srcElement.createTextRange();
                rng.moveToPoint(event.x, event.y);
            }
            else
            { //输入框
                rng = document.selection.createRange();
            }
            rng.moveStart("character", -event.srcElement.value.length);
            result = rng.text.length;
        }
        return result;
    }

    function getValue(obj) {
        var pos = getPosition(obj);
        alert(obj.value.substr(obj.value.substr(0, pos)+pos, obj.value.length));
        //alert(obj.value.substr(0, pos) + " [这里是添加的内容] " + obj.value.substr(pos, obj.value.length));
    }




</script>
 <input id="Text1" onmouseup="getValue(this)" type="text" />