The invention belongs to the field of sensor measurement, and in particular relates to a method for automatically measuring water level by adopting a constant-flow bubble type water gauge, which comprises the following steps: measuring vertical height difference H of two ports of an air duct and mounting height h0 of an air exhaust; inputting a gravity acceleration g, the height difference H of the two ports of the air duct and the mounting height h0 of the air exhaust into a data acquisition device through a register; acquiring a pressure output value P0 of the constant-flow bubble type water gauge and a gas temperature T in the duct by the data acquisition device in fixed time; and acquiring a pressure output value PA of a barometer by the data acquisition device in fixed time. The invention provides the method more accurate than classic pressure water level relation conversion formula, and discloses that the vertical height difference of the two ports of the air duct is the main reason for influencing the measuring precision of the constant-flow bubble type water gauge, but not the difference of the gravity acceleration or water density. The method can effectively reduce the water level measurement error, and has more obvious effect particularly when the vertical height difference of the two ports of the air duct exceeds 10 meters.