Abstract: Ansai oilfield Wangyao area has entered the middle and late stage of development, the formation heterogeneity is strong, water flooding is not equal, which caused difficulty of residual oil production degree in the plane and the profile. Dynamic analysis method is difficult to accurately determine the residual oil distribution law. A combination of dynamic monitoring technique is required for further analysis. Using cross-well potential to test target layer resistivity, and using resistivity distribution and change to evaluate residual oil enrichment zone. Monitoring results show that Wangyao area fresh water flooding reservoir formation resistivity followed the theory of "U" type change, that is the formation resistivity decrease firstly, then increase with the increasing of water flooding degree. High water flooding strata resistivity show rising trend, with which small parts reduce. The change is related with output water salinity, which followed the law of high salinity with low resistance and low salinity with high resistance.