Playing mahjong is a secret game with many unknown factors and strong contingency, so it enhances the gameplay. However, in the eyes of mathematicians, everything is regular and can even be expressed by mathematical formulas. A mathematician once said, "Chance is an unknown regularity." Probability theory is a branch of mathematics, which is the best annotation to this famous saying. The application of probability theory is more and more extensive, and it should also be used to interpret and guide mahjong games.