程序代码：

        private object GetElement(IHTMLDocument2 doc, string obj)
        {
            object element = doc.all.item(obj, null);
            object e = null;
            IHTMLDocument2 tmpDoc = null;
            if (element == null) //如果在当前页面的TopLevel中没有找到该元素，则到FRAME中找
            {
                for (int i = 0; i < doc.frames.length; i++)
                {
                    object oFrameIndex = i;
                    tmpDoc = (IHTMLDocument2)((IHTMLWindow2)doc.frames.item(ref oFrameIndex)).document;
                    e = GetElement(tmpDoc, obj);
                    if (e == null)
                        continue;
                    else
                        break;
                }
            }
            else
            {
                return element;
            }
            return e;
        }

        IHTMLDocument2 doc = (IHTMLDocument2)axWebBrowser1.Document;//取浏览器中网页对象
        IHTMLElement el = (IHTMLElement)GetElement(doc, "account.verifyCode");//你得知道html中输入框的id
        el.innerText="3t1b";