The 2-DOF linkage mechanism system is investigated in this paper. Based on the air-gap field of non-uniform airspace of motor caused by the vibration eccentricity of rotor, the electro- mechanical coupling relation in the real running state of motor is analyzed. And the electro- mechanical coupling nonlinear dynamic model of the system is established by the finite element method. The dynamic equation is the bases on which the subharmonic resonance properties of the system caused by self-excited inertial force effected by the actual operating frequency of the rotor of the controllable motor are analyzed by the multiple scales method. The first-order stationary solution is obtained under that condition. The work presented in the paper is benefit for further studies on the inner connections between dynamic characteristics and motors electromagnetism parameters of such mechanism system, a numerical example is presented.