Abstract: The features of old field horizontal well are large amplitude of production, rapid fluctuations in water cut and short effective time. According to those features, the paper uses graduation-geometric decline method to calibrate old field horizontal well recoverable reserves. First, decorate oil production datum when it begins to decrease obviously, and produce a new datum sequence. Selecting a new beginning point, gets decreasing ratio, and calculate production and recoverable reserves. In addition to it, test this method with the real datum of Gudong 6P1 well spacing pilot area. The applications in the other pilot areas show that the method is simple and useful without other parameters. This method is fitful for old field horizontal well to calculate recoverable reserves, and should be extended and applied.