The three-dimensional unsteady numerical method was applied to simulate the flow in the centrifugal pump with different volute casings by using shear stress transport turbulent model (SST). The base circle diameters of three volute casings were 184 mm, 198 mm and 214 mm with the same impeller, and the radial gaps between the impeller and tongue were 5.74%, 13.79% and 22.99% of the impeller radius, respectively. Different characters of the pressure fluctuation, radial hydraulic force and torque on the impeller were obtained with different gaps concerning the rotor-stator interaction. The analysis results indicated that the blade passing frequency dominated the pressure fluctuation, radial hydraulic force and torque on the impeller, of which fluctuations were periodic. The distributions of the radial hydraulic force vector coordinates were presented almost in circles with different gaps. The radial hydraulic force decreased at first then increased with the increasing volute base circle diameters. The amplitude of the pressure fluctuation on the detecting points decreased when the gaps between the impeller and the tongue became larger, and its high-frequency components were also fewer. The simulations were also presented on the characteristics of the radial hydraulic force and the tongue on the impeller of which volute base circle diameters was 184 mm at the different flow rates. For the centrifugal pump with 184 mm volute base circle diameters，its performance experiments and numeric simulations were done. It is found that the calculated results agree well with the measured results. 