In order to realize the high speed and stable operation of flexible machinery such as large overall rotating elastic beams,the Galerkin mode truncation method and Hamilton principle were comprehensively applied to establish the rigid-flexible coupling dynamic model of elastic beam.Differential equations of eccentric motion of rotor were established by rotor dynamics theory.Based on the principle of traveling wave superposition,the high-order Runge-Kutta method was used to decouple the nonlinear dynamics.Finally,the vibration mechanism of elastic beam with rotor eccentricity effect and external excitation was analyzed by time and frequency domain.The results show that under the effect of rotor eccentricity,new low-frequency harmonic components will be derived in the external excitation response;the increase in speed and excitation amplitude will lead to more low-frequency harmonic components in the frequency domain higher than the main vibration,which is easy to cause intermittent vibration of different degrees;the increase in the external excitation frequency will cause the spectral peak in the frequency domain to shift backward;a numerical simulation of the external excitation frequency band from 5 Hz to 35 Hz shows that when the frequency is 25 Hz,the low-frequency harmonic component is twice the value of the main amplitude,up to 0.181 mm.Therefore,time-frequency domain analysis can be used to effectively analyze the mechanism of sudden vibration,and provides important theoretical basis and data support for the dynamic optimization design of large overall rotating flexible machinery.