Currently, many methods for calculating oil saturation primarily rely on static methods, which are mainly used to calculate initial oil saturation and are not suitable for calculating dynamic oil saturation during the development process. To address the limitations of existing dynamic calculation methods, a general relationship between the oil?water relative permeability ratio and water saturation was derived based on the general formula for oil?water relative permeability. Through the ln(1+x) series expansion, a trinomial formula was obtained, which enables full?range fitting of the oil?water relative permeability ratio and facilitates engineering application，overcoming the drawback that previous linear formulas could only fit the middle range. By establishing the relationship between the oil?water relative permeability ratio and water cut through water cut definition and radial flow formulas, we further develop a cubic polynomial function relationship between water saturation and water cut. When the water cut of a block, single well, or single layer is known, this relationship can be used to calculate water saturation at different development stages, and then oil saturation can be derived. The research method can provide a basis for the deployment of encryption well position, fine water injection and other adjustment measures.