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By means of adopting large eddy simulation, the whole flow passage of the pump with 3-D unsteady numerical study methods was analyzed and features of fluctuation pressure of flow field inside the pump in operating states was got. The result showed that flow pattern inside the pump was complex. The frequency of fluctuation pressure was determined by rotation frequency of the pump, impeller vane number and the number of fixed guide vanes, etc. Fluctuating pressure amplitude first gradually reduced and then gradually increased along the volute weeks from the snail tongue to the volute outlet. It decreased rapidly along pumps radial from the impeller to the volute, but not changed significantly in the diffusion segment. The research method and conclusions have practical value for insuring safety operation of the super size centrifugal pump.