The tight sandstone gas reservoir of Upper Paleozoic Taiyuan Formation in the eastern margin of Ordos Basin has poor physical property and complex pore structure. Influenced by lithology and physical property, gas reservoir resistivity and rock electrical parameters vary greatly. The calculation accuracy of water saturation model based on Archie’s formulas is difficult to meet the demand. Based on three-dimensional (3D) digital numerical simulation of tight sandstone with structural variable clay minerals and variable porosity, and understanding of the law of electrical main controlling factors, the Waxman-Smits (W-S)model has been selected as the water saturation calculation model of tight sandstone gas reservoir. The W-S model parameters are calculated by logging data. The actual application shows that the absolute error of water saturation between W-S model and core analysis is less than 5%. The research results provide important technical support for the gas bearing quantitative evaluation of tight gas reservoirs.