Abstract: To study the unsteady flow characteristics caused by interaction between the impeller and volute, the unsteady flow field of a double-channel pump was simulated based on sliding mesh and RNG turbulence model. It was found that the velocity and pressure distribution in the impeller changed periodically according to the relative position of the impeller channel to the tongue. When the impeller channel was near to the volute outlet, a more regular velocity and pressure distribution was observed. The flow discharged from the impeller mixed with the re-circulated fluid in the volute at the tongue, which exhibited obvious unsteady characteristics. Pressures on the calculation monitors in the volute change periodically. Pressure fluctuations on monitors far from the volute outlet were more obvious than those near to the volute outlet, and became more and more stronger as drawing near the tongue. Comparing with the steady simulation, the calculated averaged head by unsteady simulation is more accord with the reality, which is larger than the tested head with a relative deviation of 10%.