For middle- and high- specific speed pump turbines,the hump instability appearing at large partial flow conditions seriously restricts its stable operation range. These phenomena relate closely to the inner unsteady flow in pump turbine. We investigate the characterization of pressure fluctuation and the mechanism of unsteady flow by an experimental and numerical study of the unsteady flow occurring among the adjustable blades. For pump working at either full or part load conditions,two kinds of periodic pressure pulsations in the diffuser vane have been observed and analyzed. A hump instability appears only when the actual flow rate of head curve is 0.45-0.75 times of the designed value,and our frequency analysis further shows that its appearance is closely related to the two above pressure fluctuations.