Abstract: To research the deformation situations of remaining oil film in the stage of ultra-high water cut, the deformation of oil film was investigated by means of the in fi nitesimal method based on the assumption that the oil film is displaced layer by layer. Accordingly, the main factors affecting the deformation of oil film were determined. Then, the change process of remaining oil film from the quiescent state to the critical fracturing was analyzed. Finally, the startup water flood velocity of oil film deformation and the surface area/volume ratio of oil film while its deformation reaches the critical state were theoretically derived, and the rationality of the assumption was verified by means of microscopic physical simulation experiment. It is indicated that the oil film is displaced layer by layer. As the water flood goes, the upper oil film is separated gradually from the main body, so the oil film gets thinner gradually. It is demonstrated that the oil film can be deformed and even fractured by altering the viscosity of water and the interfacial tension of oil film, so as to improve the recovery factor.