Abstract: A nonlinear spring model is used to explain the nonlinear modulation effect of micro-crack in Vibro-Acoustic Modulation (VAM).Firstly,the opening and closing states of micro-crack are equivalent to the nonlinear spring,and the nonlinear modulation phenomenon of micro-crack under VAM is analyzed theoretically.Secondly,the nonlinear spring model of micro-crack is built by finite element software,and the VAM simulation is carried out to observe the crack interface opening and closing states and the propagation characteristics of mixed excitation sound waves.Finally,the experimental study of VAM is carried out,and the nonlinear spring model is verified by using the nonlinear modulation coefficient R.The results show that the micro-crack interface opening and closing periodically with low frequency acoustic wave,makes nonlinear phenomena will be generated according to the interaction between the high frequency sound wave from crack interface and low frequency sound wave.The nonlinear modulation coefficient has the same trend with the amplitude of low frequency excitation under simulation and experiment conditions,which proves the correctness of the model.The model provides a theoretical basis for the application of VAM to ultrasonic nonlinear detection of micro-crack.