The pressure fluctuation of interflow fields in a centrifugal pump is a major factor affecting unit operating stability. The unsteady flow feature was investigated by RNG k-εturbulence model with sliding mesh technology. The pressure fluctuation and the corresponding frequency spectra at different monitoring points for different operating conditions were obtained. The results showed that the main factor, which created the pressure fluctuation in the pump, was the interaction between the impeller and the tongue, and the pressure pulsation spread widely inside the whole passage. The results also indicated that the blade passage frequency dominated for all operating conditions. The amplitude of the pressure fluctuations on the pressure side of the blade passage is larger than that on the suction side, and the pressure fluctuation was similar at the same measured surface for the outlet.