A drainage control device includes a main body, an upper cover, a lower cover, a pressure relief cover, a membrane, and a return member. The main body has a water filling trough, a drainage trough, and a water passage trough. A drainage chamber having a drainage port is defined between the upper cover and the drainage trough. A water filling chamber has a water filling port is defined between the lower cover and the water filling trough. The pressure relief cover and the main body hold a periphery of the membrane. A water passage chamber is defined between the membrane and the water passage trough. A back-pressure chamber having a drain port is defined between the membrane and the pressure relief cover. The membrane is configured to open or close a junction of the water passage chamber and the drainage chamber. The membrane is returned by the return member.