Abstract: For the analysis of the dynamic process of free piston generator (FPG), a nonlinear dynamic model that can be solved analytically was proposed, considering the load characteristics of the FPG. Energy equilibrium method was applied to achieve the first order approximate solution of the model and through which, the principle of operation and frequency characteristics of the FPG was disclosed. The FPGis a self-oscillation system with a steady limit cycle and the stable operation frequency of FPG is decided by the nonlinear stiffness and burning energy and speed of the gas in the cylinder. Moreover, the latter is proved to be a more dominant factor.