CChildFrame* pChild = (CChildFrame*)pMainFrame ->GetActiveFrame();
或者
CMDIFrameWnd *pFrame = (CMDIFrameWnd*)AfxGetApp()->m_pMainWnd;
// Get the active MDI child window.
CMDIChildWnd *pChild = (CMDIChildWnd *) pFrame->GetActiveFrame();
//or CMDIChildWnd *pChild = pFrame->MDIGetActive();
妳試壹下吧