The invention belongs to the technical field of microbubble water generating equipment, and discloses a microbubble water generating device. The microbubble water generating device comprises a sealedair and liquid mixer and a preliminary mixing device, the upper part of the air and liquid mixer is provided with an inlet, the bottom part is provided with a water draining valve, the water and air mixed preliminarily by the preliminary mixing device enters the air and liquid mixer through the inlet, an electric control stop valve is arranged at the water inlet of the preliminary mixing device, acontroller is connected with the electric control stop valve, and a water draining button is arranged on the controller. The invention further discloses an automatic water draining method of the microbubble water generating device. A user turns on the water draining valve and presses the water draining button, the controller controls to shut down the electric control stop valve, thus the water inthe air and liquid mixer can be drained out under the pressure in the air and liquid mixer, or the controller controls an air pump to introduce air into the air and liquid mixer for the preset time T1, the water in the air and liquid mixer can be rapidly drained out, while no potential safety hazards is generated to the user, and the personal safety of the user is guaranteed.