首先样式需要配合两个地方一起使用，分别是：

# 背景透明
menu.setAttribute(Qt.WA_TranslucentBackground)
# 无边框、去掉自带阴影
menu.setWindowFlags(menu.windowFlags() | Qt.FramelessWindowHint | Qt.NoDropShadowWindowHint)

其次需要注意Item中的 padding 这个值可能需要根据不同的情形进行微调

QMenu::item {
    border-radius: 4px;
    /* 这个距离很麻烦需要根据菜单的长度和图标等因素微调 */
    padding: 8px 48px 8px 16px;
    background-color: transparent;
}

完整测试代码

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""
Created on 2021/4/7
@author: Irony
@site: <a href="https://pyqt.site" title="https://pyqt.site" target="_blank" >https://pyqt.site</a> , https://github.com/PyQt5
@email: 892768447@qq.com
@file: QQMenu
@description: 
"""
import string
from random import choice, randint

try:
    from PyQt5.QtCore import Qt
    from PyQt5.QtGui import QPixmap, QPainter, QFont, QIcon
    from PyQt5.QtWidgets import QLabel, QMenu, QApplication
except ImportError:
    from PySide2.QtCore import Qt
    from PySide2.QtGui import QPixmap, QPainter, QFont, QIcon
    from PySide2.QtWidgets import QLabel, QMenu, QApplication

Style = """
QMenu {
    /* 半透明效果 */
    background-color: rgba(255, 255, 255, 230);
    border: none;
    border-radius: 4px;
}

QMenu::item {
    border-radius: 4px;
    /* 这个距离很麻烦需要根据菜单的长度和图标等因素微调 */
    padding: 8px 48px 8px 36px; /* 36px是文字距离左侧距离*/
    background-color: transparent;
}

/* 鼠标悬停和按下效果 */
QMenu::item:selected {
    border-radius: 0px;
    /* 半透明效果 */
    background-color: rgba(232, 232, 232, 232);
}

/* 禁用效果 */
QMenu::item:disabled {
    background-color: transparent;
}

/* 图标距离左侧距离 */
QMenu::icon {
    left: 15px;
}

/* 分割线效果 */
QMenu::separator {
    height: 1px;
    background-color: rgb(232, 236, 243);
}
"""


def get_icon():
    # 测试模拟图标
    pixmap = QPixmap(16, 16)
    pixmap.fill(Qt.transparent)
    painter = QPainter()
    painter.begin(pixmap)
    painter.setFont(QFont('Webdings', 11))
    painter.setPen(Qt.GlobalColor(randint(4, 18)))
    painter.drawText(0, 0, 16, 16, Qt.AlignCenter,
                     choice(string.ascii_letters))
    painter.end()
    return QIcon(pixmap)


def about_qt():
    # 关于Qt
    QApplication.instance().aboutQt()


class Window(QLabel):

    def __init__(self, *args, **kwargs):
        super(Window, self).__init__(*args, **kwargs)
        self.resize(400, 400)
        self.setAlignment(Qt.AlignCenter)
        self.setText('右键弹出菜单')
        self.context_menu = QMenu(self)
        self.init_menu()

    def contextMenuEvent(self, event):
        self.context_menu.exec_(event.globalPos())

    def init_menu(self):
        # 背景透明
        self.context_menu.setAttribute(Qt.WA_TranslucentBackground)
        # 无边框、去掉自带阴影
        self.context_menu.setWindowFlags(
            self.context_menu.windowFlags() | Qt.FramelessWindowHint | Qt.NoDropShadowWindowHint)

        # 模拟菜单项
        for i in range(10):
            if i % 2 == 0:
                action = self.context_menu.addAction('菜单 %d' % i, about_qt)
                action.setEnabled(i % 4)
            elif i % 3 == 0:
                self.context_menu.addAction(get_icon(), '菜单 %d' % i, about_qt)
            if i % 4 == 0:
                self.context_menu.addSeparator()
            if i % 5 == 0:
                # 二级菜单
                # 二级菜单
                menu = QMenu('二级菜单 %d' % i, self.context_menu)
                # 背景透明
                menu.setAttribute(Qt.WA_TranslucentBackground)
                # 无边框、去掉自带阴影
                menu.setWindowFlags(menu.windowFlags() | Qt.FramelessWindowHint | Qt.NoDropShadowWindowHint)
                for j in range(3):
                    menu.addAction(get_icon(), '子菜单 %d' % j)
                self.context_menu.addMenu(menu)


if __name__ == '__main__':
    import sys
    import cgitb

    cgitb.enable(format='text')
    app = QApplication(sys.argv)
    app.setStyleSheet(Style)
    w = Window()
    w.show()
    sys.exit(app.exec_())

效果图：