A drainage device for draining liquid out of a closed chamber includes an air cylinder, a driving assembly, a first channel, a second channel, a first sealing assembly, a second sealing assembly, and a controller. The air cylinder includes a main body defining a receiving chamber, an action piston positioned in the receiving chamber of the main body, and a connecting rod connected to the action piston. The main body defines a liquid inlet and a liquid outlet, both of which communicate with the receiving chamber, and the presence of a fixed piston with a seal in each of the inlet and outlet creates a double-acting one-way valve when the action piston is moved up and down.