The unsteady numerical method was applied to simulate the flow field and pressure field in the single-stage centrifugal pump with vane diffuser by using SST k—ω turbulent model. The static pressure recovery, energy loss and pressure fluctuation in the diffuser and volute were analyzed and the results showed that the performance using the numerical method was in good agreement with data obtained from experimental investigation. The total pressure loss and static pressure recovery in the diffuser and volute presented periodic fluctuation due to interaction between impeller and diffuser, and they were much bigger than those in volute. The pressure loss in vane diffuser was increased as the increase of flow rate. The static pressure recovery was got small as the flow rate came large. Pressure loss and pressure recovery in volute presented the same trend. The difference between the maximum and minimum pressure losses was increased as the increase of operating flow rate, as well as the pressure recovery. The pressure pulsation intensity distributed on impeller was got stronge as the increase of operating flow rate. The pressure pulsation intensity distributed on guide blade was increased at low flow rate and reached the maximum value, and then it was decreased as the increase of flow rate. The large number of vorticity in the diffuser moved to exit was induced by the vane leading edge, but it was provoked by the vane trialing in volute. The maximum pressure fluctuation intensity coefficient (Csdv) in impeller was concentrated at the impeller exit, and it was concentrated on the vane leading edge and the volute exit in the diffuser and volute, respectively.