A cooling device configured to cool a heat source includes a tank, a bellow and a solenoid valve. The tank contains a coolant, and the heat source is immersed in the coolant. The solenoid valve includes a first channel, a second channel, a third channel and a piston. The first channel is connected to the tank. The second channel is connected to the bellow. The third channel is connected to an external space. The piston is configured to seal the second channel and the third channel. The piston is configured to connect the first channel to one of the second channel and the third channel. When the heat source is initially activated, the piston is moved to connect the first channel to the third channel. When the heat source operates, the piston is moved to connect the first channel to the second channel.