Mechanism of flow instability in a centrifugal pump was studied by using the energy gradient theory. Numerical simulation was performed for the three-dimensional turbulent flow field in the centrifugal pump. The whole flow parameters in the centrifugal pump at different flow rate conditions were obtained from the simulation. The distributions of the energy gradient function K at various flow rates were calculated by using the data of the numerical simulation. According to the energy gradient method, the area with larger value of K caused instability and high turbulence intensity. The results show that instability was easier to be excited near impeller outlet area and volute tongue. In addition, as the flow rate was reduced, the unstable areas moved from the outlet to the inlet of the impeller.