Abstract: Lagrange-Maxwell's equation was extended firstly. With the theory of electromechanical analytical dynamics, the magnetic complement energy in air gap of generator was acquired. The torsional viration differential equations with periodic coefficients of rotor shafting of generator that is in the state of magnetic saturation were established. It is shown that the magnetic saturation may cause double frequency electromagnetic moment. By means of the averaging method, the first approximate solution and corresponding solution of the primary parametric resonance was obtained. The characteristics and laws of the primary parametric resonance excited by the electromagnetism were analyzed and some of new phenomena were revealed.