Abstract: Abstract: Numerical analysis of the unsteady flow in an axial flow pump was conducted to understand deeply the characteristics of turbulence in the tip region via LES (large eddy simulation) in ANSYS CFX. Such an understanding was critical to predict and eventually control cavitation and noise as well as vibration in liquid handling systems such as pumps and propellers, and improve their performance. Compared with the conventional numerical methods, LES provided the most promising and feasible alternative to compute the unsteady velocity and pressure fields. In this paper, LES method with large mesh-size requirement was used for studying the transient characteristics of the tip leakage flow and leakage vortex. Some significant conclusions were obtained by the simulation. The responsibility of different flow rate for variation of average head and efficiency calculated by LES had little discrepancy with the experimental values in the low-head axial flow pump model, especially at flow rate condition. A combination of the time-domain and frequency-domain graphs of pressure difference coefficient and leakage velocity coefficient at different chord sections were presented in this paper. It was found that the mutual promotion and restriction relationship between pressure difference and leakage velocity in the tip region was obvious, which led to the unsteady characteristics of the tip leakage flow. The tip leakage dynamics mentioned above could help others understand the underlying mechanisms of low-pressure fluctuations and tip-leakage vortex oscillations. Then, different types of tip vortexes were seen according to three-dimensional structure of leakage vortex, including the corner vortex generated by the flow separation near the pressure side, separated vortex shed from the tip into the shear layer, tip leakage vortex A formed due to the interaction between the leakage flow and the mainstream, as well as the swirl in the tip clearance. The main leakage vortex strip absorbed vortex filaments shed from the tip in the shear layer, which could provide the power for the generation of the main tip leakage vortex. The main leakage vortex was separated from the shear layer, meanwhile there was the phenomenon of "pinch off", and the main leakage vortex strip was shortened gradually because of the dissipation of its movement and the integration of the mainstream. Due to the faster transient change of the small-scale vortex in the gap compared with the tip leakage vortex, the re-generation cycle of vortex filaments in shear layer was shortened, and then the secondary leakage vortex strip was created above the main vortex strip. From the in-plain leakage vortex structure with mean streamlines and vorticity contours, it could be seen that the separation vortex in shear layer constantly was separated and then rolled up by the leakage vortex with the increasing of chord coefficient, which could drive the main leakage vortex to move forward, however, the vorticity of the main leakage vortex decreased during the moving process towards the pressure surface of the neighboring blade and the counter-rotating induced vortexes were generated constantly, at the same time, the scope of the main leakage vortex expanded a lot, and a large fraction of the wake downstream of the leakage vortex were produced, which could affect the flow field in the passage and enhance the instability of the flow field.