In order to analyze the internal flow characteristics of a double volute centrifugal pump, the CFD software CFX was applied to simulate the unsteady flow among different working conditions based on standard k—ε turbulence model and standard no slip mesh model. The pressure fluctuation characteristics under different working conditions on flow channels both in the impeller and the volute were obtained. The results showed that pressure pulsation of each monitoring point was relatively large and uniform under the condition of low flow rate. Pressure fluctuation near the edge of impeller outlet was 5~8 times stronger than that in other region of the impeller. In the case of rotating 30 steps (90°) and 90 steps (270°) during four working conditions, the value of pressure reached to maximum. Pressure fluctuation in the inner flow passage was stronger than that in the outer flow passage of the double volute, and it reached to maximum in the tongue of the double volute. The pressure fluctuation decreased firstly and then increased near the tongue and outlet of volute in the large flow rate due to a certain back-flow. It can be concluded from fast Fourier transform that the frequency of the impeller and its multiples were main frequencies of pressure fluctuation which presented a decreasing tendency.