The axial pump impellers with blade tip clearances of 0.15，0.50，1.00，2.00 mm were simulated in the multi-condition by CFD software based on standard k-ε turbulence model and SIMPLEC algorithm, using structured grid technology. The influence of different tip clearances on the performance of the axial pump and the meridional velocity, and circulation were discussed. The formation of leakage flow and leakage vortex was investigated with different tip clearance sizes.When the blade tip clearance is more than 0.50 mm, numerical results show that the decline trend of the pump performance significantly increases, the axial velocity near the hub and blade tip clearance decreases, and the circulation increases . The larger of blade tip clearance, the greater impact of the leakage flow on axial velocity and circulation. By numerical simulation, leakage flow and leakage vortex under different blade tip clearances are captured, and the analysis come to the conclusion that the tip clearance leakage vortex is formed by leakage flow entraining with mainstream and its trace is closely related to the tip clearance size.
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