#include <QtGui>
#include <QtWidgets>
...
QDesktopWidget desktop;
QRect rect = desktop.availableGeometry(this);
QPoint center = rect.center();
int x = center.x() - (width()/2);
int y = center.y() - (height()/2);
center.setX(x);
center.setY(y);
move(center);
...