In this paper, the nonlinear dynamic characteristics of vehicle semi-active suspension system with SMA spring were studied in hysteretic nonlinear theory. SMA spring was applied in semi-active suspension system to control vibration. Von del Pol hysteretic cycle model were introduced to set up a new kind of continuous SMA strain-stress model, based on which the nonlinear dynamic model of vehicle semi-active suspension system with SMA spring was developed. The first-order nonlinear approximate solution of suspension system was obtained, the stability and bifurcation characteristic of suspension system were analyzed. The result of analysis shows that the nonlinear stiffness parameters can not cause the bifurcation of suspension system, and the qualitative change of the dynamic characteristic of suspension system has relationship with the nonlinear damping parameters. Finally, the result of analysis was proved by simulation.