//获取文本框光标位置

//获取文本框光标位置

function getTxtCursorPosition(txt) {

    var cursurPosition = -1;

    if (txt.selectionStart) {//非IE浏览器

        cursurPosition = txt.selectionStart;

    } else {//IE

        try {

            var range = document.selection.createRange();

            if (range.text.length == txt.value.length) {

                cursurPosition = 0;

            } else {

                range.moveStart("character", -txt.value.length);

                cursurPosition = range.text.length;

            }

        } catch (e) {

            cursurPosition = 0;

        }

    }

    return cursurPosition;
}

 <textarea id="target">fdajklfj al</textarea>

        <p><button id="btn">插入话题</button></p>

        <script language="javascript">

            var $ = function (id) {

                return document.getElementById(id);

            };

            $("btn").onclick = function () {

                var con = "请在这里输入自定义话题";

                //转载文字

                $("target").value += "#" + con + "#";

                var l = $("target").value.length;

                //创建选择区域    

                if ($("target").createTextRange) {//IE浏览器

                    var range = $("target").createTextRange();

                   // range.moveEnd("character", -l)

                    //range.moveStart("character",-l)              

                    range.moveEnd("character", l - 1);

                    range.moveStart("character", l - 1 - con.length);

                    range.select();

                } else {

                    $("target").setSelectionRange(l - 1 - con.length, l - 1); //

                    $("target").focus();

                }

            };

        </script>