Abstract: Based on the Maxwell equations and elastic dynamics theory, the electrodynamics equation and the nonlinear magneto-elastic coupling vibration equation of a current-conducting thin plate in a longitudinal magnetic field are deduced. By Galerkin method, the Duffing type vibration equation of the current-conducting strip plate with its two opposite sides simply supported is obtained. To the subharmonic resonance, the method of Multiple Scales is used to solve the equations. The corresponding approximately analytical solution, the amplitude-frequency response equation and the existential condition of nontrivial solutions are derived. According to Lyapunov stability theory, the stability of the solution is analyzed, and the discriminant of steady solutions is obtained. By numerical calculations, the existential region of nontrivial solutions, amplitude characteristic curves and phase trajectories in a moving phase plane are obtained. The effects of electro-magnetic and mechanical parameters on system resonance characteristics are analyzed.