1.获取屏幕的坐标系
2.获取窗口的坐标系
3.移动窗口，使其坐标为width = （屏幕坐标系.width - 窗口坐标系.width）/ 2 。使其height = （屏幕坐标系.height - 窗口坐标系.height）/2

import sys
from PyQt5.QtWidgets import QDesktopWidget, QMainWindow, QApplication


# 定义一个类，继承主窗口“QMainWindow”
class CenterForm(QMainWindow):
    # 在构造函数init中初始化
    def __init__(self):
        # 调用父类
        super(CenterForm, self).__init__()
        # 设置主窗口标题
        self.setWindowTitle('让窗口居中')
        # 设置窗口尺寸
        self.resize(400, 300)

    def center(self):
        # 获取屏幕坐标系
        screen = QDesktopWidget().screenGeometry()
        # 获取窗口坐标系
        progrem_size = self.geometry()
        newLeft = (screen.width() - progrem_size.width()) / 2
        newtop = (screen.height() - progrem_size.height()) / 2
        self.move(newLeft, newtop)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    main = CenterForm()
    main.show()
    sys.exit(app.exec_())