1.添加头文件：#include <Windows.h>
2.添加库：LIBS +=User32.LIB   
win32 {
    LIBS += -luser32
}

3.添加相关代码：
    QString ClassName( "YodaoWordBookClass" );
    QString AppName( tr("有道单词本" ));
    LPCWSTR App = reinterpret_cast <LPCWSTR>(ClassName.data());
    LPCWSTR AppCaption = reinterpret_cast <LPCWSTR>(AppName.data());
    //亦可不用上面两名，直接转换即可，ClassName.utf16(), AppName.utf16())
    if(HWND AppWnd=::FindWindow(App,AppCaption))
    {
        ::ShowWindow(AppWnd,SW_SHOWNORMAL);
    }
    else
    {
        ui->youDaoTitle_lineEdit->setText(tr("单词本打开失败"));
    }

LRESULT CALLBACK WindowProc(
        HWND hwnd,      // handle to window
        UINT uMsg,      // message identifier
        WPARAM wParam,  // first message parameter
        LPARAM lParam   // second message parameter
        )
{
    if( uMsg == 10000)
    {
        int kk = 0;
    }
    return DefWindowProc( hwnd, uMsg, wParam, lParam );
}

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    WNDCLASS wc;
    memset(&wc, 0, sizeof(wc));
    wc.style = 0;
    wc.lpfnWndProc = &WindowProc;
    wc.cbClsExtra = wc.cbWndExtra = 0;
    wc.hInstance = NULL;
    wc.hIcon = NULL;
    wc.hCursor = NULL;
    wc.hbrBackground = NULL;
    wc.lpszMenuName = NULL;
    wc.lpszClassName = L"xfs Notify Window";
    RegisterClass(&wc);
    m_hWnd = CreateWindow(wc.lpszClassName, L"WWWWWWWWWWWW",
                          WS_DISABLED, 0, 0, 0, 0, NULL, NULL, wc.hInstance, NULL);

    SendMessage(m_hWnd, 10000, 0, NULL);

    int ok=0;
}