function insertText(obj,str) {
    if (document.selection) {
        var sel = document.selection.createRange();
        sel.text = str;
    } else if (typeof obj.selectionStart === 'number' && typeof obj.selectionEnd === 'number') {
        var startPos = obj.selectionStart,
            endPos = obj.selectionEnd,
            cursorPos = startPos,
            tmpStr = obj.value;
        obj.value = tmpStr.substring(0, startPos) + str + tmpStr.substring(endPos, tmpStr.length);
        cursorPos += str.length;
        obj.selectionStart = obj.selectionEnd = cursorPos;
    } else {
        obj.value += str;
    }
}

function moveEnd(obj){
    obj.focus();
    var len = obj.value.length;
    if (document.selection) {
        var sel = obj.createTextRange();
        sel.moveStart('character',len);
        sel.collapse();
        sel.select();
    } else if (typeof obj.selectionStart == 'number' && typeof obj.selectionEnd == 'number') {
        obj.selectionStart = obj.selectionEnd = len;
    }
} 

使用：
onclick="insertText(document.getElementById('text'),'新文字')"
onclick="moveEnd(document.getElementById('text'))"