Abstract: In order to determine the distribution state of micro residual oil in different oil displacement stages and guide the residual oil tapping strategy of old oilfields in the later stage of water injection development, cores with different levels of permeability in typical blocks are selected for oil displacement experiments, and the displacement stage is divided into three parts: saturated oil stage, water drive stage to the initial water stage of the model, and the stage after water drive, The liquid nitrogen freezing technology is used to make core slices and the high-pressure mercury lamp fluorescence microscope is used to analyze the microscopic remaining oil occurrence state in different displacement stages. After image processing, six types of remaining oil occurrence states are subdivided: membrane, particle adsorption, corner, throat, cluster and inter particle adsorption, The remaining oil proportion in different states was calculated. The water drive of medium permeability core mainly uses the free remaining oil. After water drive, there is more free remaining oil, which can be used as the object of further tapping potential; During water flooding of low permeability cores, the content of free micro residual oil is further reduced. After water flooding, bound membrane residual oil and semi bound throat residual oil can be used as replacement development objects. The fluorescence microscopic observation method of high-pressure mercury lamp provides a new quantitative method for residual oil analysis and reference for the residual oil tapping strategy of old oilfields.