| this->resize(150,150);  
 QDesktopWidget* desktop = QApplication::desktop();
 int width = desktop->width();
 int height = desktop->height();
 move((width - this->width())/2, (height - this->height())/2);
 
 | 
PS:resize要放在调整窗体位置前面,不然刚开始的this->width()和this->height()是默认的,再调整就不能保证是居中了