In　order　to　provide　reference　for　loading　carrying　capacity　evaluation　and　maintenance　strategy,　the　damage　detection　is　performed　f　or　long　span　cable-stayed　bridge.　Based　on　the　structure　feature　of　cable-stayed　bridge,　the　main　girder　is　considered　as　a　beam　on　elastic　foundation,　furthermore,　the　internal　force　redistribution　only　occurs　on　the　elements　near　the　damage.　Therefore,　the　position　of　damage　can　be　detected　according　to　the　change　of　cables　forces　in　the　same　load　case.　Based　on　the　reference　FE　model,　the　optimization　method　is　employed.　Taking　the　degree　of　the　damage　elements　as　the　design　variables　and　the　difference　of　cable　forces　between　the　measured　value　and　calculated　value,　the　FE　model　is　updated　in　order　to　identify　the　degree　of　the　damage　elements.　Because　of　the　linear　relationship　between　the　internal　force　redistribution　and　the　initial　structure　deformation,　the　changes　of　cable　forces　under　the　same　truck　load　are　sensitive　to　the　structure　damage.　Finally,　taking　the　Sutong　Bridge　as　an　example,　four　damage　cases　are　analyzed　and　identified.　The　calculation　results　show　that　using　the　reference　FE　model　and　the　changes　of　cable　forces　under　the　same　truck　load,　the　position　and　the　degree　of　the　girder　damage　can　be　identified.