A method of exchanging gas in a process of making oxygenated water is performed in a water tank. The water tank has a water inlet, a water outlet, a gas inlet, and a gas outlet. Water is received in the water tank. The water outlet and the gas inlet are closed, and the gas outlet is opened. And then supply the water tank with water through the water inlet to make a water level in the water tank go up and make gas above the water level escape through the gas outlet. Next, close the gas outlet and open the gas inlet and the water outlet once the water tank is almost filled up. Finally, close the gas inlet and the water outlet when the water level is lower than a predetermined level.
