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In order to study the rotating stall characteristics of axial-flow pump, the unsteady internal flow field in a large axial-flow pump was numerically simulated. The Pro/E software was used to build the pump model, which mainly consisted of inlet pipe, impeller, guide vane and outlet pipe. The unsteady flow was numerically simulated based on RANS solver and SST k-ω turbulence model. The real pump pressure pulsation was tested by high frequency dynamic pressure sensor. The streamlines and pressure pulsation in pump at different flow rates were explored. The experimental pressure pulsation results and the numerical results were close to each other, which illuminated the reliability of the numerical calculation method. The results showed that there were two reversed flow field areas in the leading edge closely to shroud and trailing edge closely to hub of blade suction surface under rotating stall condition. The dominant frequency under designed condition in the impeller inlet and the middle of guide vane was blade passing frequency. The dominant frequency under designed condition in the impeller outlet was guide passing frequency because of rotor-stator interaction. The pressure pulsation coefficient amplitude was markedly increased under deep rotating stall condition. The pressure pulsation coefficient amplitude of monitoring point G6 at guide vane outlet under deep rotating stall condition was 16 times larger than that under designed condition. Low frequency pressure pulsation at 0.83Hz disappeared under deep rotating stall condition which was caused by a vortex in guide vane. The research had reference value for the safe and steady operation of pump unit in large pumping station.