In order to study the effects of pressure fluctuation under different thickness of the volute tongue in a centrifugal pump, the unsteady three-dimensional numerical simulation was computed by adopting Reynolds-averaged equations closed by the RNG k-ε model and the sliding-mesh method．The inner flow characteristics and pressure fluctuation were obtained, and the corresponding frequency spectra analysis was conducted. The results show that under design condition,the hydrodynamic forces on impellers are related to the pressure fluctuation affected by the interaction between the impeller and the volute tongue, the frequency of hydrodynamic forces on impellers is the impellers passing frequency through the volute tongue; The amplitude of hydrodynamic forces coincide with the increase of the thickness of volute tongue.