Abstract: In order to clarify the distribution of microscopic remaining oil in different oil displacement stages and guide the exploration of remaining oil potential in the late stage of water injection development in mature oilfields, cores with different permeability levels from typical blocks were selected for oil displacement experiments. Liquid nitrogen refrigeration technology was used to prepare core slices, and high-pressure mercury lamp fluorescence microscopy was used to analyze the microscopic remaining oil occurrence status during the saturated oil stage, the initial water breakthrough stage, and the post water flooding stage. Through image processing, the occurrence state of remaining oil were further categorized, and the proportions of remaining oil in different states were calculated. The results show that water flooding in medium-permeability core mainly utilizes free-state residual oil, and there is more free residual oil remaining after water flooding, which can be used as a potential target for further exploration. The free-state microscopic residual oil in low-permeability cores was further reduced during the water flooding process, and then the membrane-shaped residual oil in bound state and the throat-shaped residual oil in semi-bound state after water flooding can be used as alternative development resource. The microscopic observation method by high-pressure mercury lamp fluorescence provides a new quantitative approach for the remaining oil analysis and a good reference for the exploration of remaining oil potential in mature oilfields.