In view of the combined differential surge chamber with the connecting pipe, the shaft and the differential upper chamber, the effects of differential hole flow coefficient combination on the performance of surge chamber and pressurized water diversion system was studied by numerical simulation according to a long diversion power station project. Through analysing the calculation results of surge change process of the surge chamber, the maximum speed increase rate of the unit and the pressure at the end of the volute, we concluded that the upper chamber and riser can fully play the role and then improve the performance of the surge chamber, only in proper differential hole flow coefficient combination. Our conclusion has practical significance to further study the hydraulic transition process of the combined differential surge chamber and a positive effect on the application of the surge chamber in practical engineering.