Using the shear stress transport turbulent model （SST), the 3-D unsteady numerical method was applied to simulate the flow of the centrifugal pump with the common-tongue and step-tongue. Various pressure fluctuations, radial hydraulic force on the impeller, and volute were obtained for the rotor-stator interaction. The analysis results indicated that the blade-passing frequency dominated the pressure fluctuation, the radial hydraulic force on the impeller and volute, which had periodic fluctuations. The amplitudes of the pressure fluctuation on the inlet and outlet detecting points of the pump were smaller with step-tongue than with common-tongue; the high-frequency components were also fewer. The radial hydraulic force on the volute of the pump with the step-tongue and its amplitude were also smaller than with the common-tongue. The distribution of radial hydraulic force on the impeller vector coordinates were almost circular using common-tongue and step-tongue.