The cavitation flow field in the axial flow model pump with tip clearance δ=0.5 mm was simulated based on modified SST k-ω turbulence model and cavitation model. The cavitation bubble distributions at different circumferential sections, meridional velocity distribution at impeller outlet and cavitation volume fraction were analyzed. Numerical and experimental results showed that the NPSH curve and tip region cavitation flow were predicted successfully. Cavitation inception in axial flow pump occured in the tip region, including blowing vortex cavitation, leakage flow cavitation, rolling region cavitation and tip leakage vortex cavitation. In the condition of cavitation number σ=0.451, the tip leakage vortex induced obvious cavitation cloud. The cavitation range of rolling region expanded with the decreasing of cavitation number gradually, and merged with the leakage flow cavitation area, and finally extended the cavitation bubble cloud to the entire suction side of the blades. The meridional velocity at impeller outlet near the gap region decreased gradually, in the influence of the gap leakage flow, and was further declined with the decreasing cavitation number. In different cavitation conditions, the circumferential section cavitation area coefficient of the blade, ranging from the hub to the tip, increased primarily, nd decreased afterwards. In the tip region, cavitation area coefficient bursted to the peak and dropt rapidly.