In the paper, according to the theory of oil drainage ellipsoid in horizontal wells, the control-volume of horizontal wells was the volume of ellipsoidal truncated cone that was truncated by the top interface and bottom interface of the oil layer, and the geological reserve of horizontal wells is the weight of oil and gas in the control-volume. The theoretical formula of the geological reserve of horizontal wells in volumetric method is deduced according to the theory, and the geological reserve of 2-P25 Well was calculated by the formula and the result shows that a high precision difference is 6.8%. The method can be widely used in oil production.