Due　to　the　development　of　the　Wide　Area　Measurement　System　(WAMS),　it　has　been　possible　to　obtain　the　real-time　information　of　the　power　system.　But　in　some　research　fields　such　as　transient　stability　analysis　and　emergency　control,　the　real-time　information　is　not　enough.　Thus　study　on　trajectory　prediction　is　necessary　and　important.　In　this　paper,　a　trajectory　real-time　prediction　scheme　based　on　the　WAMS　information　is　proposed　for　the　multi-machine　system.　We　firstly　analyse　the　features　of　angle,　angular　velocity　and　unbalance　power　trajectory　in　a　Single　Machine　Infinite　Bus　(SMIB)　system.　Based　on　the　analysis,　we　choose　different　prediction　methods　for　different　state　variables.　Then　we　transform　the　multi-machine　system　into　an　equivalent　SMIB　system.　By　clustering　and　equivalence,　we　obtain　the　trajectory　of　equivalent　system.　And　we　propose　the　real-time　trajectory　prediction　scheme　based　on　above　analysis.　Finally　we　test　the　real-time　prediction　scheme　on　the　IEEE　standard　system.　The　results　show　the　accuracy　and　stability　of　the　prediction　scheme　proposed　in　this　paper.