Remaining saturation is a core parameter for reserve assessment, development plan optimization and enhanced oil recovery in the middle and later stages of traditional reservoir development. Its calculation accuracy is directly limited by the ability to characterize the heterogeneity of geological models in numerical simulation. To determine the specific distribution location and amount of remaining oil, this paper proposes a calculation method of remaining saturation based on the fine digital-analog algorithm. By improving the mathematical model of numerical simulation and based on the conventional coarse model, combined with the Beckle-Lewilt equation oil displacement theory. By using the relationship between the derivative of water saturation fw' and the water passage multiple PV, the oil saturation of each fine grid in each layer grid of the fine model is derived to achieve high-precision characterization of the fluid migration process in the reservoir. Take a certain block of Jidong Oilfield as an example, the specific distribution location and amount of remaining oil were calculated through the fine numerical model. This calculation method of remaining oil saturation based on the fine model can effectively improve the accuracy of remaining oil prediction and can calculate the specific distribution location and amount of remaining oil. The calculation method of the remaining oil in the fine model is of great significance for determining the specific location and amount of the remaining oil.