The hydraulic conditions of hyropower stations with long diversion channels and high water head are complicated, it is necessary to clarify the extreme pressure conditions in the spiral case of the power units under centralized water supply arrangement and the intrinsic influencing mechanism. Assuming the water levels of the upstream and downstream of the hydropower plant are the same, we carried out simulation calculations for the transient process of the load rejection conditions of single and double power units under different loads using the method of characteristics (MOC), Simplex which can track the unit operation trajectory and the system state equation. The results show that the single unit and double units operating at 20% of the rated power are subjected to first-phase water hammer after load rejection, which is the extreme pressure condition in the inlet section of the spiral case. The study also shows that the extreme value of water pressure in the inlet section of the spiral case occurs in the case of partial load rejection when the guide vane operates at small opening. The initial pressure value and the extreme value in the inlet section of the spiral case are related to the unit load, the less the initial output of the unit, the smaller the head loss in the diversion system, the higher the initial pressure of the unit, and the higher the value of the water pressure in the inlet section of the spiral case after load rejection, and the slower the pressure decay after rejecting partial load. The operating mode of the power units has a certain effect on the extreme value of water pressure in the inlet section of the spiral case and its peak time. The research results can provide some reference for the design, operation and maintenance of the same type of hydropower stations.