Abstract: In water flooding reserves calculation,the empirical formula'7.5 B'of Tong Xianzhang is generally adopted.While in application,the calculation result by this formula is always bigger,sometimes it is even bigger than the OOIP and then this formula can not be used in specific reservoirs.Starting from one-dimension two-phase flow,the derivation on Mr.Tong's B-type water drive has been carried out.Through water drive curve,a new method based on relative permeability and water drive reserves has been established.The new method can overcome the absence of relative permeability data and make the calculated water drive reserves more reasonable.The application proves this new method is of application value for offshore oil field development indexes evaluation.