The invention discloses an oil drainage device of a hydraulic oil tank. The oil drainage device of the hydraulic oil tank comprises an oil tank body, a base and a plug. The base is fixed to the bottomof the oil tank body and communicates with the interior of the oil tank body; and the plug is installed in the base through threads, the oil tank body, the base and the plug form a sealing cavity body, and connecting pieces communicating with the interior of the base are arranged on the side surface of the base. The oil drainage device of the hydraulic oil tank has the beneficial effects that theplug is used for sealing, the tail of the plug and the cone-shaped ring surface of the base form reliable linear sealing, the plug is only loosened during oil drainage, the oil drainage amount is adjusted according to the back-out length of the plug, the oil drainage plug is sealed and reliable, the operation is simple, processing and manufacturing are easy, the oil drainage is convenient, and the maintenance efficiency is greatly improved.