<a href="javascript:void(0)" onclick="a()">回复他</a>
<textarea id="t" cols="100" rows="10">abc</textarea>

<script>
  function a(){
    var e=document.getElementById("t");
    e.focus();
    e.innerText="回复1楼：\r\n";
  }
</script>

------解决方案--------------------
HTML code

<a href="javascript:doit('回复1楼');void(0);">回复他</a>
<textarea id="ta" rows="10" cols="40">xxx</textarea>
<script language="JavaScript"> 
function doit(s){
    var t = document.getElementById("ta");
    t.value = s + "\n";
    t.focus();
    if(document.all) t.createTextRange().collapse(false);
    else t.selectionStart = t.value.length;
}
</script>