The maximum upsurge (MU) and the maximum air chamber pressure (MACP) are critical parameters for the design of air cushion surge chamber (ACSC) in hydropower stations. In this paper, the existence of the MU and the MACP are proved under compound conditions. The theoretical formula predicting the most dangerous superposition moment of the MU and the MACP under compound condition is derived, and the influence factors are analyzed as well. To verify the accuracy of the formula, the rigid model based on Runge-Kutta method (RKM) and the elastic model based on the method of characteristics (MOC) are established, respectively, according to the parameters of the ACSC system in the practical hydropower station. The numerical results agree well with the theoretical predictions. In addition, the MU and the MACP under three control conditions are simulated, respectively, and the results show that when the cross-sectional area of throttled orifice is small, the MU and the MACP occur under the successive load rejection condition (SLR); when the cross-sectional area is large, the MU and the MACP occur under the load rejection after load acceptance condition (LRLA).