Abstract: The S characteristics of a high-head pump turbine working in turbine mode could cause difficulty in power grid connection, and in severe cases it may induce vibrations of the generating unit, producing an adverse effect on safe and stable operation of the power station. To study pressure fluctuations in the pump turbines working in the S zone, we focused on a pump turbine at a pumped-storage power station and numerically simulated its steady and unsteady flows. The characteristics of pressure fluctuations in the vaneless sections and draft tube were analyzed for the cases of stable operation, small torque, small discharge, and reverse pump condition. The results show that the fluctuation in static pressure is significant and its behaviors depend on the compressibility of water. In the vaneless section between guide vane and runner, the dominant frequency is equal to the one generated by blades passing when the compressibility is considered, while it is equal to 2fn in the small torque case when incompressible water is assumed. The fluctuating amplitudes are slightly larger in the compressible case, but in both compressible and incompressible cases the characteristic amplitude is increased and then decreased as the unit discharge is decreasing. And the amplitudes at high-frequencies such as 14fn and 21fn are relatively small. In the draft tube, pressure fluctuations are featured with dominant frequencies equal to fn, 3fn and 4fn, but their amplitudes become the smallest in stable operation. In the elbow and outlet section, the amplitudes in other conditions could be more than ten times larger than those in stable operation, showing the effects of spiral vortex structure.