The invention discloses a method and a device for testing the in-situ pressure osmosis of a coating. The device consists of a pressurizing chamber which can be attached to the coating, and an air compression system which provides required atmospheric pressure for the device, wherein the pressurizing chamber is a rigid cylinder, and one end of the pressurizing chamber is open to form a coating attachment end; a water inlet and outlet valve is arranged near the coating attachment end; and a liquid level scale ruler which is used for observing the liquid level in the chamber is arranged on the pressurizing chamber. The device for testing the in-situ pressure osmosis of the coating can perform non-destructive waterproof in-situ test on the waterproof performance of the coating on the basis of not damaging the structural coating so as to directly reflect the actual condition of the waterproof performance of the coating, is particularly suitable for performing in-situ test after the coating is constructed or used for certain time, and has the advantages of convenience in operation and quick detection.