Abstract: In order to investigate the influence of cantilever ratios of rotor on the hydraulic vibration of the nuclear main pump, 25 model schemes for the CAP1400 nuclear main pump were designed by altering the cantilever ratio of the pump shaft under the condition that other geometric parameters were determined. Based on Reynolds time averaged N-S equations and RNG k-ε turbulence model in multiple reference frames, full three-dimensional numerical calculation was carried out for reduced-scale models of nuclear main pump with solid coupling method, the variation law of stress and strain of the impeller blade of the nuclear main pump was analyzed under the different cantilever ratios and working conditions. The rotor dynamics characteristics of the nuclear main pump with different cantilever ratios were investigated under the conditions with pre-stress and without pre-stress. The results reveal that when the cantilever length is fixed, the stress and strain of the impeller blade and the first 6 order natural frequencies of rotor have little correlation with the cantilever ratio, while having great correlation with the working condition. The maximal deformation of the impeller blade of the nuclear main pump increases with the length of the cantilever, whereas not changing with the cantilever ratio. The maximal stress does not change with the cantilever length and the cantilever ratio. The fluid solid interaction can reduce the natural frequency of the rotor system, and the magnitude of reduction decreases with the order number increasing.