In　this　paper,　the　global　bifurcation　for　a　rotor-active　magnetic　bearings　(AMB)　system　with　the　timevarying　stiffness　is　investigated　through　discussing　the　zeros　of　the　Melnikov　function.　Some　critical　conditions　of　chaos　occurring　are　given.　According　to　the　results,　the　case　that　the　order　of　Melnikov　function　is　more　than　or　equal　to　2　is　more　general　than　other　case　(no　zero　or　simple　zero).　In　other　word,　homoclinic　tangency　may　occur　in　several　resonance　relations.　©2009　IEEE.