看代码：

var  document: IHTMLDocument2;
  ole_index: OleVariant;
  frame_dispatch: IDispatch;
  frame_win: IHTMLWindow2;
  frame_doc: IHTMLDocument2;
begin
//获取主文档  
document := WebBrowser1.Document as IHTMLDocument2;
  
  if not Assigned(document) then
    Exit;
//获取第二个frame，为IDIspatch类型
  ole_index := 1;
  frame_dispatch := document.Frames.Item(ole_index);

  //如果不为空，转换IDispatch为IHTMLWindow2,获取document
  if frame_dispatch <> nil then
  begin
    frame_win := frame_dispatch as IHTMLWindow2;
    frame_doc := frame_win.document;
  end;

接下来就可以根据frame_doc进行各种处理了。关键就是通过IDispatch中转类型。