Abstract: In order to study the principle of the water reflow phenomenon in the Mariotte bottle, the calculation formula for the amount of the water reflow is derived by using the Ideal Gas Law and the theories of pressure in this paper. And the validity of the formula is verified by experiments. It is found that the calculated value of the supplementary water amount is basically consistent with the actual value, which shows that the formula is applicable. At the same time, the results show that the returned water enters not only the intake pipe, but also the Mariotte bottle. The water reflowed to the Mariotte bottle will compress the air above the water surface and reduce the volume of the cavity in the bottle, which further reveals the mechanism of the water reflow process in the Mariotte bottle.