The invention provides an automatic ventilation device of an oil tank. The device comprises a first floating board, a second floating board and a movable valve, and a movable valve; a first floating board comprises a through hole, peripheral edge of the through hole is provided with a first enclosing body, and the first enclosing body is provided with a first upper flange; the second floating plate is provided with a second enclosing body and a second upper flange is arranged on the second floating plate; the movable valve comprises a plate body, the outer peripheral wall and the inner peripheral wall are arranged on the plate body, and the outer wall is provided with a first lower flange which is interfered with the first upper flange, and the inner wall is provided with a second lower flange which is interfered with the second upper flange, a first ventilation hole is arranged on the upper part of the outer peripheral wall, and a second ventilation hole is arranged on the upper part of the inner peripheral wall; and a movable valve is covered on the through hole, so that the outer peripheral wall is combined in the inner diameter of the first enclosing body, and the inner peripheral wall is joined in the inner diameter of the second enclosing body. The automaticl ventilation device can be used for smoothly injecting oil or absorbing oil; and the device has a simple strucutre in order to avoid mechanical damage, and the device has high sensitivity and low usage cost.