Core experimental analysis method, logging interpretation method and empirical formula method are the main methods to determine the irreducible water saturation of clastic rock reservoir. Due to cost and other factors, most evaluation units are unable to obtain irreducible water saturation with core analysis method, which can only be obtained by logging interpretation method or empirical formula method with relatively low accuracy. To solve this problem, based on the analysis of the formation mechanism of irreducible water saturation in clastic rock reservoir, the specific surface area of clastic rock and the surface water film thickness of clastic particles are determined using the analysis items such as clastic rock particle size, clastic rock bulk density, clay minerals and whole rock X-ray diffraction. The irreducible water saturation equation based on the parameters of clastic rock particle diameter, clastic particle mass fraction, rock bulk density and clay mineral mass fraction is established. The accuracy of using this equation to explain the irreducible water saturation of the evaluation unit is higher than that by logging interpretation method, and is equivalent to that determined by core indirect analysis method in core experimental analysis. The equation reveals that the irreducible water saturation of clastic rocks is directly proportional to the product of water film thickness and specific surface area, and inversely proportional to the total porosity of rocks, which lays a theoretical foundation for the evaluation of irreducible water saturation of clastic rocks. The study has practical implications for the determaination of irreducible water saturation of clastic rock with lower costs and high precision.