Abstract Cavitating flows in a mixed-flow pump are numerically simulated using CFD software based on SIMPLEC algorithm and structured-mesh. The standard SST k-ω turbulence model and homogeneous flow cavitation model are adopted to investigate the influence of tip clearance on cavity stability in the impeller and obtain the optimum range of tip clearance. The results show that the blade tip clea-rance has a significant effect on the cavitation characteristics in the mixed-flow impeller; specially, the turbulence scale and intensity of tip leakage vortex increase but the critical cavitation number decreases gradually with increasing tip clearance under part-load flow conditions. When the critical cavitation number is 0.357, a larger tip clearance can suppress the inception of cavitation on blade surfaces. When severe cavitation occurs at 0.123 cavitation number a smaller tip clearance can inhibit large scale bubbles on blade surfaces. At a small tip gap, these bubbles are attached to blade suction side and become stable. With increase of tip clearance, the cavity shedding area is migrated to the primary flow zone and the bubbles are accumulated in the middle of blade suction side, and the vapor vo-lume fraction and cavity thickness are increased gradually.