In order to obtain the cavitation performance and force on runner of pump-turbine in pump mode, the model pump-turbine in a pumped storage power station was selected as the study object. Based on SST k-ω turbulence model and Zwart cavitation model, the numerical simulation was carried out for fully-three-dimensional unsteady flow field with different cavitation coefficients and compared with the experiment results. Turbulent kinetic energy distribution on the middle stream surface of runner and guide vanes, the vapor volume and loading distribution on blades was discussed, and then force on runner was quantitatively analyzed. Results showed that the numerical analysis results can be used to capture development of the cavitation;with the decrease of the total pressure in draft inlet, the turbulent kinetic energy was increased gradually in outlet of the runner and decreased gradually in outlet of guide vanes, vapors generated at suction side of blade near the leading edge at first and then extended to the exit of runner and pressure side of blade;due to rotorstator interaction about runner and guide vanes and asymmetry of the volute’s shape, periodicity and symmetry of variation of radial force on the runner was gradually destroyed. The research findings can reveal the cause of operation became worse for pump-turbine in pump mode.