To study the dynamics of giant magnetostrictive material (GMM) transducer, its model was developed, according to the dynamic experiment results of GMM and the influence of unsymmetrical piece-wise linear stiffness of the pre-pressing spring. Based on the one degree of freedom its vibration model of a GMM transducer, unsymmetrical piecewise linear nonlinear characteristic of pre-pressing spring was investigated. The first order harmonic motion component of the GMM transducer was obtained by the analysis of KBM method. By the numerical simulation the complicated bifurcation and chaos behavior of the nonlinear vibration system were founded, which should to be taken account of in the design of GMM transducer.