In order to study the reasonable size of the main and auxiliary surge chambers in the upstream series double surge chambers system ( the main surge chamber is impedance type and the auxiliary surge chamber is simple type ), the characteristic line method and the transition process software were used to calculate and study the transition process of the pressure water diversion system. The variation law of maximum surge water level, fluctuation attenuation rate and maximum water hammer pressure at the end of volute in different size combinations of main and auxiliary surge chambers were analyzed. The results showed that the influence of the impedance hole of the main surge chamber on the water hammer pressure decreased with the increase of the size of the auxiliary surge chamber when the double surge chambers system was arranged. The size of the impedance hole had a critical point below which the maximum surge water level of the auxiliary surge chamber decreases with the increase of the size of the impedance hole and rises above this point. The size of auxiliary surge chamber was too large and the fluctuation attenuation of double surge chambers was slow. The results are useful for similar projects to select reasonable sizes of the main and auxiliary surge chambers.