The non-uniform disturbance in the circumferential direction is the essential cause of the occurrence of rotating stall in a turbomachinery. In order to study the effect of tip clearance leakage flow on rotating stall, the mixed-flow pump models with different tip clearances were numerically calculated, and then the energy performance curves and internal flow structures were obtained and compared. The results show that the pump efficiency and the internal flow field of the numerical calculation are in good agreement with the experimental results. A saddle region appeared in the energy performance curves of the three tip clearances, and with the decrease of clearance, the head and efficiency of the mixed-flow pump increased to varying degrees, but the critical stall point shifted to the large flowrate, and the stable operating range of the mixed-flow pump decreased, which indicating that the mixed-flow pumps were easier to stall under small tip clearance. Under the deep stall condition, the influence of the leakage flow on the end wall area increases gradually with the decrease of clearance. Under the small clearance, the leakage flow flows away from the suction surface for a certain distance to form a number of tip leakage vortex strips with the mainstream action, overflows the leading edge of the next blade, and then flows downstream into different flow passages, aggravating backflow and secondary flow separation at the blade inlet, which seriously damaged the spatial structure of the inlet flow. This resulted in the earlier occurrence of stall. With the increase of tip clearance, the tip leakage vortex develops along the radial direction to the middle of the flow channel and serious flow separation phenomenon occurs in the downstream channel, which induces the deep stall. Under the 0.8 mm tip clearance, the whole impeller outlet passage is almost blocked by the backflow of the guide vane inlet, and a deep stall was induced.