The unsteady flow in a mixed-flow pump was simulated based on RANS solver embedded with SST turbulence model and SIMPLEC algorithm. The time-domain and frequency-domain of pressure fluctuations at four representative monitoring surfaces under different flow rate and rotational speed conditions were analyzed. Comparison between averaged calculated power and test data verified that interior flow simulated by this numerical model was relatively accurate. The numerical results showed that the maximum pressure fluctuation appeared at impeller inlet where the pressure fluctuation amplitude increased from hub to shroud. The dominant frequency at impeller inlet and impeller outlet was the impeller blade passing frequency, but the dominant frequency at middle and outlet of the stator was correlative with the flow rate. The pressure fluctuation became stronger as deviating from the optimum flow rate, especially at impeller inlet when small flow rate conditions. The optimum flow rate conditions of different rotational speed had a similar dominant frequency of pressure fluctuation, the pressure fluctuation amplitude increased with the rotational speed increasing.