When the bottom-water reservoir is developing, the well will be flooding by the rising cone of the bottom water. The research of flooding range is of great significance for developing remaining oil in bottom water reservoir. This paper uses the actual production data of oil well to return the C-type water flooding characteristic curve; then, get the sweep efficiency of different water cut according to the regression result; on the other hand, the cone range is equivalently represented as a geometry; finally, the flooding range formula of vertical well and horizontal well in bottom-water reservoir is obtained by mathematical calculation. This formula reflects the dynamic production characteristics of oil well, compared with the conventional static method, which is more real responsive to the flooding range of oil well. Taking a sand of the Bohai oilfield Q as an example, the results calculated by this formula accurately reflect the residual oil distribution of the sand, which is consistent with the numerical simulation results.