When Archie formula is applied to calculate water saturation of the reservoir, the cementation factor is usually taken a fixed value. In fact, the cementation factor of different reservoir is not the same, and it changes with the pore structure and shale content. As the cementation factor is controlled by pore structure and shale content, first of all, we must make use of core-analysis data and GR or SP to establish the formula that can calculate the cementation factor, and then use the well logging data according to this formula to determine the cementation factor in each reservoir. In this way, making the calculated result approximately the same with the real value of the cementation factor could make sure that the Archie formula can be used to calculate water saturation of reservoir with high accuracy.