Abstract: Changing magnetic chain and voltage as generalized coordinates of a discrete electromechanical coupling system, kinetic energy, potential energy, electrical energy, magnetic energy and dissipation function are obtained. The concept of the pondermotive force of the electromechanical coupling system is introduced. By means of the circuit equation and power equilibrium equation, the expression of the pondermotive force is derived. The Lagrangian energy function of the electromechanical coupling system is brought in, and the dynamical equation is aquired based on generalized coordinates, magnetic chain and voltage. It suits nonlinear condition of the circuir of the electromechanical coupling system.