Abstract: In numerical simulation of hydraulic transient process of pumped storage power station, due to the special nature of four-quadrant characteristic curve of pump-turbine, the minimum pressure at the inlet of the draft tube will be greatly reduced under the condition of successive load rejection and often become a control value. Based on the method of characteristics (MOC) of one dimensional rigid water hammer and elastic water hammer, a pumped storage power station that adopts the scheme of “two turbines sharing one diversion tunnel” was taken as an example, and the influence of diameter of its diversion tunnel and the relative position of its tail water bifurcated pipe on transition process under successive load rejection were numerically simulated. The results show: with the arrangement of tailrace surge tank, it is not economical to adjust the minimum pressure at the inlet of the draft tube by changing the tunnel diameter; The tunnel area is optimal when it keeps the flow rate at an economical level;Under the premise of meeting the requirements of engineering layout, the length of the tail water bifurcated pipe should be reduced as much as possible to ensure the safe operation of water conveyance and power generation system.