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Based on turbulence model of shear stress transport(SST) k-ω, homogeneous multiphase model, and Zwart cavitation model, numerical simulations for high specific speed axial flow pumps with three difference blade number impellers were conducted. External characteristic curves and head performances of cavitation were obtained. Through the analysis, critical NPSHc of three impellers, whose blade number were 3, 4, 5, was respectively 6.19m, 5.122m and 4.765m. In severe cavitation area, the more blades the impeller had, the faster head declined with NPSH reducing. Aiming at the two cavitation type in axial flow pump: fixed cavitation on suction side and vortex cavitation in tip region, the cavitating distribution rule in different NPSH value was found. Compared among three impellers, in the same NPSH, the five blades impeller got smaller cavitating distribution on suction side, which meant lower cavitation degree. However, this impeller would get a sharp head decline when it came to severe cavitation area. Low pressure zone caused by tip leakage vortex moved and expanded to upstream with the decreasing number of impeller blades. Moreover, cavitating volume in the impeller with smaller blade number was larger than the others, and it indicated that this impeller’s cavitation degree was severer than others. So the effect of blades number on different cavitation types and different NPSH was not the same.