Abstract: In order to analysis and improve the head prediction accuracy of a centrifugal pump at a low flow rate, the inner flow field in a centrifugal pump, whose specific speed is 64, is calculated by steady multi-phase simulation and unsteady numerical simulation respectively. The computational zone includes pump inlet extension, impeller, volute, shroud chamber, hub chamber, and pump outlet extension. The Reynolds Averaged Navier-Stokes approach is used to solve the control equations. The grid of wear ring is refined and the whole gird number independency is checked. The roughness height is considered in the simulation and is set to be 25 μm. The inlet boundary condition is pressure inlet and the outlet boundary condition is velocity outlet. The RNG k-ε turbulence model is applied in the simulation. The convergence accuracy is set to be 10-4. The effects of the phase angle in a steady multi-phase numerical simulation and the time step in unsteady numerical simulations on the head prediction are analyzed. The comparison between the steady multi-phase simulation and unsteady numerical simulation is discussed and analyzed. The results show that the phase angle has only a little impact on the head prediction, while the time step has a great influence on the head prediction. The prediction results of unsteady numerical simulation are better than that of steady multi-phase simulation, whose errors are all below 2%. As the flow rate decreases, the rotor-stator interaction between impeller and volute get more obvious. Therefore, it is necessary to consider the rotor-stator interaction when the pump head is predicted by CFD.