#textarea {

    display: block;

    margin:0 auto;

    overflow: hidden;

    width: 550px;

    font-size: 14px;

    height: 18px;

    line-height: 24px;

    padding:2px;

}

textarea {

    outline: 0 none;

    border-color: rgba(82, 168, 236, 0.8);

    box-shadow: inset 0 1px 3px rgba(0, 0, 0, 0.1), 0 0 8px rgba(82, 168, 236, 0.6);

}

/**

* 文本框根据输入内容自适应高度

* @param                {HTMLElement}        输入框元素

* @param                {Number}                设置光标与输入框保持的距离(默认0)

* @param                {Number}                设置最大高度(可选)

*/

var autoTextarea = function (elem, extra, maxHeight) {

        extra = extra || 0;

        var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,

        isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),

                addEvent = function (type, callback) {

                        elem.addEventListener ?

                                elem.addEventListener(type, callback, false) :

                                elem.attachEvent('on' + type, callback);

                },

                getStyle = elem.currentStyle ? function (name) {

                        var val = elem.currentStyle[name];

                        if (name === 'height' && val.search(/px/i) !== 1) {

                                var rect = elem.getBoundingClientRect();

                                return rect.bottom - rect.top -

                                        parseFloat(getStyle('paddingTop')) -

                                        parseFloat(getStyle('paddingBottom')) + 'px';

                        };

                        return val;

                } : function (name) {

                                return getComputedStyle(elem, null)[name];

                },

                minHeight = parseFloat(getStyle('height'));

        elem.style.resize = 'none';

        var change = function () {

                var scrollTop, height,

                        padding = 0,

                        style = elem.style;

                if (elem._length === elem.value.length) return;

                elem._length = elem.value.length;

                if (!isFirefox && !isOpera) {

                        padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));

                };

                scrollTop = document.body.scrollTop || document.documentElement.scrollTop;

                elem.style.height = minHeight + 'px';

                if (elem.scrollHeight > minHeight) {

                        if (maxHeight && elem.scrollHeight > maxHeight) {

                                height = maxHeight - padding;

                                style.overflowY = 'auto';

                        } else {

                                height = elem.scrollHeight - padding;

                                style.overflowY = 'hidden';

                        };

                        style.height = height + extra + 'px';

                        scrollTop += parseInt(style.height) - elem.currHeight;

                        document.body.scrollTop = scrollTop;

                        document.documentElement.scrollTop = scrollTop;

                        elem.currHeight = parseInt(style.height);

                };

        };

        addEvent('propertychange', change);

        addEvent('input', change);

        addEvent('focus', change);

        change();

};

<textarea id="textarea" placeholder="回复内容"></textarea>

    <script>

        var text = document.getElementById("textarea");

        autoTextarea(text);// 调用

    </script>