In　this　paper,　we　use　the　asymptotic　perturbation　method　to　analyze　the　nonlinear　dynamics　of　a　rotor-active　magnetic　bearing　(AMB)　system　with　16-pole　legs.　The　motion　governing　equation　is　derived　by　using　classical　Newton　law.　The　resulting　dimensionless　equation　of　motion　for　the　system　is　expressed　as　a　two-degree-of-freedom　system　including　the　parametric　excitation,　quadratic　and　cubic　nonlinearities.　The　asymptotic　perturbation　method　is　used　to　obtain　the　averaged　equation　when　the　primary　resonance　and　1/2　sub-harmonic　resonance　are　taken　into　consideration.　From　the　averaged　equations　obtained,　numerical　simulations　are　presented　to　investigate　the　modulation　of　vibration　amplitudes　＂of　the　rotor-AMB　system.　Based　on　a　specific　set　of　parameters,　it　is　found　that　there　exist　the　periodic,　quasi-periodic　and　chaotic　motions　in　the　modulated　amplitude　of　the　rotor　in　the　system.