Flow instability in a miniature centrifugal pump is numerically simulated with the RANS equations and the SST k-ω turbulence model. The energy gradient method is adopted to analyze the flow instability at design load and two off-design loads, and the results are compared with those analyzed by Q-criterion. The regions with large magnitude of energy gradient function (K) indicate pronounced turbulent intensity and poor flow stability. Internal flow stability is investigated in details for both the near blade surfaces region and the impeller passages.To study the mechanism of energy gradient method, internal flow parameters such as the velocity and total pressure, the transverse gradient of total mechanical energy and the work done by shear stresses are investigated respectively. The results show that the energy dissipation reaches its maximum around the leading edge of suction surface. The value of the energy gradient function K presents a different magnitude for the near blade surfaces region and the impeller passages, and the K in the impeller passage is much larger. Regions with maximum of the work done by shear stresses are concentrated on the suction surface, regions with large transverse gradient of total mechanical energy is concentrated on the hub surfaces or shroud surfaces. It is further found that the K can reflect the influence of the outer boundaries of vortex on the flow near blade surface.