The invention discloses an automatic drainage device of an air reservoir and a control method thereof. The device comprises the air reservoir, an automatic drainage valve and a hand-pulling water discharge valve, a water discharge port of the air reservoir is connected with the automatic drainage valve, and the water discharge port of the air reservoir is connected with the hand-pulling water discharge valve through a tee coupling. According to the automatic drainage device of the air reservoir and the control method, only the automatic drainage valve is additionally arranged on the basis of an original bus pneumatic brake, and only an ECU and a drainage switch are additionally arranged on the basis of an original circuit; operation is easy, and use is convenient.