The　circular　truss　antenna　of　the　large　aperture　is　considered　to　be　a　flexible　structure　which　may　cause　vibration　in　space　and　may　affect　its　performance.　The　frequency　analysis　of　the　circular　truss　antenna　is　an　important　problem　for　understanding　its　vibration　mechanism.　In　this　paper,　we　investigate　the　frequency　characteristics　of　a　beam-ring　structure　which　is　proposed　for　the　first　time　to　model　the　circular　truss　antenna　in　the　case　of　the　antenna　expended　and　locked.　Based　on　describing　the　displacements　of　the　beam-ring　system　in　detail,　the　kinetic　energy　and　potential　energy　are　calculated.　The　partial　differential　governing　equations　of　motion　and　boundary　conditions　for　the　beam-ring　structure　are　derived　by　Hamilton　principle.　From　the　linear　parts　of　the　governing　equations　of　motion　and　the　corresponding　boundary　conditions,　the　linear　frequencies　of　the　beam-ring　structure　are　theoretically　obtained.　The　effects　of　the　physical　parameters　on　the　frequency　characteristics　of　the　beam-ring　structure　are　studied,　which　are　further　verified　by　the　numerical　results.　The　finding　phenomena　of　this　paper　are　helpful　for　designing　and　controlling　the　beam-ring　structure　such　as　the　circular　truss　antenna.