方法一：

在新建ui文件時不要新建Dialog

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file ‘D:\pythonProjects\pyqtUITest\untitled6.ui‘
#
# Created by: PyQt5 UI code generator 5.11.3
#
# WARNING! All changes made in this file will be lost!

from PyQt5 import QtCore, QtGui, QtWidgets
 
from PyQt5.QtCore import *

class Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(188, 250)
        self.exit_game = QtWidgets.QPushButton(Form)
        self.exit_game.setGeometry(QtCore.QRect(0, 200, 189, 51))
        self.exit_game.setObjectName(
 
"exit_game")
        self.new_game = QtWidgets.QPushButton(Form)
        self.new_game.setGeometry(QtCore.QRect(0, 50, 189, 51))
        self.new_game.setObjectName("new_game")
        self.return_to_mainMenu = QtWidgets.QPushButton(Form)
        self.return_to_mainMenu.setGeometry(QtCore.QRect(0, 
 
150, 189, 51))
        self.return_to_mainMenu.setObjectName("return_to_mainMenu")
        self.continue_button = QtWidgets.QPushButton(Form)
        self.continue_button.setGeometry(QtCore.QRect(0, 0, 189, 51))
        self.continue_button.setObjectName("continue_button")
        self.ranking_list = QtWidgets.QPushButton(Form)
        self.ranking_list.setGeometry(QtCore.QRect(0, 100, 189, 51))
        self.ranking_list.setObjectName("ranking_list")

        Form.setWindowFlags(Qt.FramelessWindowHint)

        self.retranslateUi(Form)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "Form"))
        self.exit_game.setText(_translate("Form", "退出遊戲"))
        self.new_game.setText(_translate("Form", "新遊戲"))
        self.return_to_mainMenu.setText(_translate("Form", "返回主菜單"))
        self.continue_button.setText(_translate("Form", "繼續遊戲"))
        self.ranking_list.setText(_translate("Form", "排行榜"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    Form = QtWidgets.QWidget()
    ui = Ui_Form()
    ui.setupUi(Form)
    Form.show()
    sys.exit(app.exec_())

方法二：在建的Dialog模板中傳入MainWindow的參數

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file ‘Sub_menu.ui‘
#
# Created by: PyQt5 UI code generator 5.11.3
#
# WARNING! All changes made in this file will be lost!

from PyQt5 import QtCore, QtGui, QtWidgets
# from PyQt5.QtCore import *
# from PyQt5.QtWidgets import *
from PyQt5.QtCore import Qt
class Ui_Dialog(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(189, 250)
        # self.setWindowFlags(Qt.FramelessWindowHint)
        # self.setWindowFlags(Qt.FramelessWindowHint)
        self.pushButton_3 = QtWidgets.QPushButton(Dialog)
        self.pushButton_3.setGeometry(QtCore.QRect(0, 0, 189, 51))
        self.pushButton_3.setObjectName("pushButton_3")
        self.pushButton_4 = QtWidgets.QPushButton(Dialog)
        self.pushButton_4.setGeometry(QtCore.QRect(0, 50, 189, 51))
        self.pushButton_4.setObjectName("pushButton_4")
        self.pushButton_6 = QtWidgets.QPushButton(Dialog)
        self.pushButton_6.setGeometry(QtCore.QRect(0, 100, 189, 51))
        self.pushButton_6.setObjectName("pushButton_6")
        self.pushButton_7 = QtWidgets.QPushButton(Dialog)
        self.pushButton_7.setGeometry(QtCore.QRect(0, 150, 189, 51))
        self.pushButton_7.setObjectName("pushButton_7")
        self.pushButton_8 = QtWidgets.QPushButton(Dialog)
        self.pushButton_8.setGeometry(QtCore.QRect(0, 200, 189, 51))
        self.pushButton_8.setObjectName("pushButton_8")

        Dialog.setWindowFlags(Qt.FramelessWindowHint)
        self.retranslateUi(Dialog)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.pushButton_3.setText(_translate("Dialog", "繼續遊戲"))
        self.pushButton_4.setText(_translate("Dialog", "新遊戲"))
        self.pushButton_6.setText(_translate("Dialog", "排行榜"))
        self.pushButton_7.setText(_translate("Dialog", "返回主菜單"))
        self.pushButton_8.setText(_translate("Dialog", "退出遊戲"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_Dialog()
    ui.setupUi(MainWindow)   #這裏傳入了一個MainWindow的參數
    MainWindow.show()
    sys.exit(app.exec_())

這裏我喜歡使用第一種方法解決Dialog界面會默認選中一個選項的問題。

pyQT Dialog默認選中某一個選項問題的解決