In　this　paper,　a　double-disc　rotor　system　with　two　support　rolling　bearings　is　reduced　by　the　proper　orthogonal　decomposition　(POD)　method.　The　finite　element　method　(FEM)　is　used　to　establish　the　rotor　system　model　(96DOFs).　The　basic　theory　of　the　POD　method　and　the　concrete　expression　in　the　rotor　system　is　introduced.　The　rotor　system　is　reduced　to　12,　5,　and　2　DOFs　by　the　POD　method,　respectively.　The　time　history　curves,　and　frequency　spectrums　of　the　reduced　and　original　systems　are　compared　for　verifying　the　accuracy　of　the　POD　method.　The　2-DOFs　reduced　system　is　solved　by　the　Harmonic　balance　and　alternating　frequency/time　domain　(HB-AFT),　whose　results　are　in　good　agreement　with　those　by　the　Runge-Kutta　method.