To study the influence of cavitation coefficients of pump turbine on the inner flow characte-ristics under the partial load condition, the unsteady numerical simulation was carried out by using the RNG <i>k-ε</i> turbulence model and Schnerr-Sauer cavitation model, based on the model pump turbine in a pumped storage power station. And the hydraulic performance and inner flow state of the unit at different cavitation stages were analyzed. The results showed that the occurrence and hydraulic efficiency decreased after cavitation developed and the axial force increased significantly. Under the partial load condition, cavitation vortex strip originated in the runner cone and motioned slowly in spiral type toward to the downstream.With the decrease of the cavitation coefficients, vortex strip gradually changed from fine to coarse. Coverage of the low-pressure area extended outward, and the velocity changed more rapidly in the radial direction. There was a serious second backflow in the movement process of the cavitation vortex strip. The main stream advanced on the downstream along the cone tube wall.This is extremely easy to hit the wall and causes the strong hydraulic vibration, which adversely affects the operation security and stability of the unit.