The flow in end wall region of axial-flow impeller has an important impact on flow structure, energy transfer and hydraulic efficiency. The hydraulic axial-flow pump models with different tip clearance sizes were simulated based on CFD technology and high-quality structured grid. The influence of tip clearance on the meridional velocity and circulation distributions in impeller inlet and outlet were discussed, and the leakage vortex generation mechanism and its structure were analyzed as well. The numerical results were compared with high-speed photography experimental results. The investigation results showed that tip clearance led to non-uniform meridional velocity distribution in impeller inlet and secondary flow in end wall region occurred, meridional velocity near tip clearance in impeller outlet decreased, and the bigger the gap was, the bigger decline of meridional velocity would be. The secondary recirculation near the wheel led to circulation of impeller inlet, when the gap increased to 2mm, about 50% of the whole flow passage appeared circulation which impacted by the tip leakage vortex. The trajectory and structure of tip leakage vortex in the numerical simulation showed agreement with the experimental results. Under the small flow rate conditions, the leakage vortex strength and the influence regional scale were increased.