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In order to grasp the formation and evolution mechanism of the tip leakage vortex of axial flow pump, and evaluate the influence of the cavitation condition of vortex formation and the gap width, an axial flow pump clearance leakage flow experiment and numerical calculation were carried out. The three-dimensional visualization of the gap flow characteristics and the vortex structure was obtained by applying the streamline vortex cloud diagram. The distribution of the velocity streamlines, vorticity and turbulent kinetic energy on the suction surface were analyzed as the vortex at the initial generation. The physical quantity distributions of different cross-sections were compared, and the relationship between cavitation development and TLV vortex strength was analyzed for different cavitation conditions. Results showed that the leakage shear zone was the main area of TLV formation,where the turbulent kinetic energy and vorticity were large. Convection formed by the axial mainstream and interstitial jets promoted the generation and development of vortices. The intensity and scale of vortices under large clearance were larger, accompanied by greater leakage velocity.The vortex in the core area of TLV came from the shear vortex formed by the shear zone and the circumferential incoming vortex.With a large cavitation number, the vortex and cavitation distribution were basically the same, the vortex strength was positively correlated with cavitation, and the tip vortex cavitation extended farther in the large gap.With a small cavitation number, the vortex and the cavitation position did not completely overlap, and the gap width had little effect on cavitation. Due to the lower environmental pressure, the vortex strength required for cavitation formation was smaller, so it was easy to form cavitation and expand to form sheet cavitation. The TLV vortex strength was increased along the leading edge of the blade, reaching the maximum near the trailing edge, while the average vortex strength was the largest at the leading edge of the blade, where cavitation occurred first.