这可以通过将QMenu子类化并将弹出菜单移动到showEvent中您想要的位置来完成：

弹出菜单.h

#ifndef POPUPMENU_H
#define POPUPMENU_H

#include <QMenu>

class QPushButton;
class QWidget;

class PopupMenu : public QMenu
{
    Q_OBJECT
public:
    explicit PopupMenu(QPushButton* button, QWidget* parent = 0);
    void showEvent(QShowEvent* event);
private:
    QPushButton* b;
};

#endif // POPUPMENU_H

弹出菜单.cpp

#include "popupmenu.h"
#include <QPushButton>

PopupMenu::PopupMenu(QPushButton* button, QWidget* parent) : QMenu(parent), b(button)
{
}

void PopupMenu::showEvent(QShowEvent* event)
{
    QPoint p = this->pos();
    QRect geo = b->geometry();
    this->move(p.x()+geo.width()-this->geometry().width(), p.y());
}

主窗口.cpp

...
PopupMenu* menu = new PopupMenu(ui->pushButton, this);
...
ui->pushButton->setMenu(menu);

它看起来像这样：

另一种（恕我直言）更简单的方法是：

void MainFrame::Slot_ShowMenu()
{
    auto pMenu = new QMenu(this);

    connect(pMenu, &QMenu::aboutToHide, pMenu, &QMenu::deleteLater);

    ...

    // Retrieve a valid width of the menu. (It's not the same as using "pMenu->width()"!)
    int menuWidth = pMenu->sizeHint().width();

    int x = mUI.myQPushButton->width() - menuWidth;
    int y = mUI.myQPushButton->height();

    QPoint pos(mUI.myQPushButton->mapToGlobal(QPoint(x, y)));

    pMenu->popup(pos);
}

你应该为你的QMenu实现一个eventFilter，在eventFilter方法中，你需要计算菜单显示的位置。
这里有一个例子：

.赞成

TEMPLATE = app

QT     += widgets
SOURCES += main.cpp \
           dialog.cpp

HEADERS += dialog.h

FORMS   += dialog.ui

主.cpp

#include <QtWidgets/QApplication>

#include "dialog.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Dialog dia;
    return dia.exec();
}

对话框.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QtWidgets/QDialog>
#include <QMenu>
#include "ui_dialog.h"

class Dialog : public QDialog
{
    Q_OBJECT
public:
    Dialog();

protected:
    bool eventFilter(QObject * obj, QEvent *event);

private:
    QMenu *menu;
    Ui::Dialog m_ui;
};

#endif

对话框.cpp

#include "dialog.h"

Dialog::Dialog()
{
    m_ui.setupUi(this);

    menu = new QMenu("menu", this);
    menu->installEventFilter(this);
    QAction *action = new QAction("action#1", this);
    menu->addAction(action);
    m_ui.pushButton->setMenu(menu);
}

bool Dialog::eventFilter(QObject * obj, QEvent *event)
{
    if (event->type() == QEvent::Show && obj == m_ui.pushButton->menu())
    {
        int menu_x_pos = m_ui.pushButton->menu()->pos().x();
        int menu_width = m_ui.pushButton->menu()->size().width();
        int button_width = m_ui.pushButton->size().width();

        QPoint pos = QPoint(menu_x_pos - menu_width + button_width,
                            m_ui.pushButton->menu()->pos().y());

        m_ui.pushButton->menu()->move(pos);
        return true;
    }
    return false;
}