Comparing with conventional hydraulic turbine (e.g. Francis turbine), pump turbine shows significant unstable characteristics because its design is a compromise between a pump and a turbine. In present paper, unsteady flow and pressure fluctuations within a model pump turbine are numerically studied through Partial Averaged Navier Stokes (PANS) approach. The PANS approach is fulfilled through modification of RNG k-e turbulence model in a commercial CFD code. Pump turbine operating at different conditions with guide vanes opening angle 6° is simulated. Results revealed that the predictions of performance and relative peak-to-peak amplitude by PANS approach agree well with the experimental data. Velocity, vortex and turbulent kinetic energy at the inlet of runner are very large near the pressure surface and the blade leading edge, leading to high pressure fluctuations within the vaneless area otablef pump turbine. The maximum amplitude of pressure fluctuation occurs when the pump turbines run at runaway point. The primary dominant frequency of pressure fluctuation is the runner blade passing frequency in the vaneless space. The above high pressure fluctuations should be avoided during the design of pump turbines especially those operating at high-head condition.