Abstract: The governing equation of a rotating flexible ferromagnetic disk subjected to AC electromagnet is modeled. The equation is discretized by Galerkin method, and effects of the disk rotating speed, AC frequency and electromagnetic force stiffness on the natural frequency and stability of the system are analyzed. The analysis shows that the stiffness results in a parametric instability of the disk when the AC frequency approaches to the natural frequency combination or the rotating speed approaches to a critical speed of the system. And with the increase of the stiffness, the disk could be unstable in a larger range of the disk rotating speed and AC frequency. The damping of disk material can be increased to suppress the instability.