In this paper, the fine/turbo software of NUMECA was used. Based on the three-dimensional steady Reynolds averaged N-S equations, the S-A turbulence model was used to calculate the relationship between tip clearance and diameter of the impeller, which were δ = 0.1%, 0.3%, 0.5% and 0.7% respectively. The clearance flows of four water jet axial flow pumps with different tip clearances were numerically studied at different flow rates. The results indicate that the size of the clearance affects the cavitation performance of the blades, the efficiency and head of the pump, and has a great impact on the performance of the pump. It provides a useful exploration for the design improvement of axial-flow waterjet pump.