In order to understand the effect of the ring clearance size on the internal flow characteristics of a high speed multistage deep well pump, based on SST k-ω turbulence model and refined mesh, the influence of the ring clearance on the internal flow field of the pump was studied. Based on the entropy generation theory, the characteristics of leakage flow loss in the pump under different ring clearance were analyzed in detail. The results show that with the increase of the ring clearance size, the total head and efficiency of the multistage pump show decreasing trends. The increase of leakage makes the flow pattern at the inlet of the impeller worsen. However, when the ring clearance size increases to a certain range, the increase of the leakage can slow down. Turbulent dissipative entropy production is the main part of the flow loss, and the cavity and impeller inlet are the major loss zones. The ring clearance size has little effect on the vortex characteristics in the cavity, but has an obvious effect on the vortex strength in the impeller inlet and impeller channel.