A drainage device includes a tank, a pipe and an air duct. The tank has a base plate and at least one first wall. The first wall is disposed on the base plate. The base plate and the first wall define a space. The pipe defines a channel. The pipe connects with the base plate. The channel communicates with the space. The air duct is disposed partially in the space and partially in the channel. There exists at least one gap between an outer surface of the air duct and an inner surface of the pipe.