Abstract: Simandoux Equation and Indonesia Equation, with the function of mud correction, have been widely used in saturation logging interpretation of argillaceous sandstone. Study shows that, as to the Simandoux Equation, the accuracy of saturation calculation is limited due to the limitation of assumed conditions and the parameters selected in the model, although the influence of mud additional conductivity on the formation resistivity can be eliminated by mud correction. On one hand, the conductive ability isn't distinguished between clay and fine silt sand by Simandoux Equation; On the other hand, the rock-electrical parameters are generally empirical values or obtained by rock-electrical experiment instead of the values corrected after eliminating the influence of mud additional conductivity. In order to solve the shortages mentioned above and emphasize the role of clay in the mud additional conductivity, an improved Simandoux Equation is proposed and, on the basis of this, a method for estimating the clay content by GR curve is summarized. Thus, rock-electrical parameters without the influence of mud additional conductivity are obtained, based on the rockelectrical experiment data and multiple regression model fitting. The verification conducted in X gas field of East China Sea Shelf Basin shows that the saturation calculated by the modified Simandoux Equation is more precise than that of by Simandoux Equation.