Abstract: Evaluation of residual oil saturation is the difficulty and key in the water flooding zone logging interpretation.Wettability and formation water resistivity will change a lot once the reservoir is flooded.Therefore some parameters used in the saturation estimation need to be redefined.One example which lie in fluvial facies deposit oil field of South China Sea for studying key parameters in saturation evaluation of flooding zone is given in the paper.An effective way of getting key parameters for residual oil saturation evaluation was developed through the rock electricity experiments simulating reservoir conditions and relationship pursue between the mixed water salinity and the actual water cut.The method was in great accordance with field practice in application,and lays a foundation for exact evaluating residual oil saturation of such kinds of oilfield development.