The cross-sectional size of an air-cushion surge chamber is usually determined by the critical stability cross-sectional area, and the hydrostatic head, the flow through the generating set, and the thermodynamic process of gas in a surge chamber when there is little perturbation of the water level are three main factors affecting the critical area. Investigations of the heat conduction process between the gas in the surge chamber and the surroundings show that the isothermal change (with a polytropic exponent of 1.0) is most appropriate for simulating the actual conditions when there is little perturbation. Through mathematical analysis of a formula regarding the critical area and a case study, it is concluded that using the maximum hydrostatic head during the normal operation of a hydropower station and the corresponding flow as the calculation conditions can satisfy the fluctuation stability requirement of the water level in the surge chamber of other operation schemes , which would facilitate the safety of projects.