In　this　paper,　the　nonlinear　dynamics　of　a　rotor-active　magnetic　bearing　system　with　16-pole　legs　and　the　time　varying　stiffness　is　investigated.　The　magnetic　forces　are　obtained　through　an　electromagnetic　theory.　The　motion　governing　equation　is　derived　by　using　Newton　law.　The　resulting　dimensionless　equation　of　motion　for　the　rotor-AMB　system　with　16-pole　legs　and　the　time　varying　stiffness　is　presented　with　the　two-degree-of-freedom　system　including　parametric　excitation,　the　quadratic　and　cubic　nonlinearities.　The　averaged　equations　of　the　rotor-AMB　system　are　obtained　by　using　the　method　of　multiple　scales　under　the　case　of　the　primary　parametric　resonance　and　1/2　sub-harmonic　resonance.　The　numerical　results　show　that　there　exist　the　periodic,　quasi-periodic　and　chaotic　motions　in　the　rotor-active　magnetic　bearing　system.　Since　the　weight　of　the　rotor　effect　the　system,　it　is　also　found　that　there　are　the　different　shapes　of　motion　on　the　two　directions　of　the　rotor-AMB　system.　The　parametric　excitation,　or　the　time-varying　stiffness　produced　by　the　PD　controller　has　great　impact　on　the　system.　Thus,　the　complicated　dynamical　response　in　the　rotor-AMB　system　can　be　controlled　through　adjusting　the　parametric　excitation.　©　Copyright　2017　ASME.