It is of great significance to clarify the variation law of oil-water flow relationship and the ultimate oil displacement efficiency in the high water cut stage of the oilfield to guide the deep potential tapping of the oilfield in the later stage of water flooding. Through the nuclear magnetic-CT collaborative experiment, based on the core displacement experiment, it is proposed to use the fractal dimension to describe the change law of pore throat structure in different water flooding stages. The fracture shape calculation of the core in various water flooding phases is obtained, which is based on the relationship between pore throat size and capillary pressure in porous media. Combined with the B-L equation and the welgee quation, the variation law of the oil-water relative permeability ratio curve considering the microscopic heterogeneity of the reservoir is obtained. A set of relative permeability experimental calculation methods that can describe the complete water flooding process is established, and the ultimate oil displacement efficiency evaluation is further completed. As there was more water flooding than before, the fractal dimension of the reservoir rises from 2.46 to 2.73. After considering the change of core physical properties, the ultimate oil displacement efficiency of water flooding is reduced by 3.92% on average. Considering that the relative permeability curve and water drive characteristic curve corrected by fractal theory can accurately predict the development performance of the reservoir after the change of reservoir physical properties, the ultimate oil displacement efficiency of the well area is evaluated finely, and the oilfield is guided to carry out differential adjustment and potential tapping in the high-water cut stage.