In drainage for coast and reservoir bank slopes by using a siphon drainage method, there exists a defect of difficult self-restoring of siphonic action because of reduction in vacuum degree in a siphon after the siphonic action pauses due to groundwater recession. The present invention provides a drainage device capable of activating siphonic action in real time. The device is composed of an open-type dam, an air chamber, a siphon, an exhaust pipe, an air replenishing pipe, and a one-way valve. The air chamber is disposed at the crest of the open-type dam; one end of the siphon extends below the phreatic line of a slope, and the other end of the siphon extends through the air chamber and below the seawater level; one end of the exhaust pipe extends below the phreatic line of the rear edge of the slope, and the other end of the exhaust pipe is connected to the air chamber; and the air replenishing pipe is disposed at the upper end of the air chamber. The device provides power for the siphonic action with guarantee for self-restoring of the siphonic action. Moreover, the device employs a pile foundation with low consumption of materials and low construction costs in the face of great depth of water before a slope, and thus is applicable to not only coast slopes but also reservoir bank slopes with extra-deep water levels.