It　is　important　to　estimate　the　longitudinal　fundamental　period　of　cable-stayed　bridges　accurately　in　the　selecting　of　structure　system　and　evaluating　of　seismic　response　of　a　cable-stayed　bridge.　A　reverse　double-mass　model　to　compute　the　longitudinal　fundamental　period　of　a　floating　cable-stayed　bridge　is　proposed　based　on　the　transfer　paths　of　seismic　horizontal　inertia　forces　under　longitudinal　seismic.　The　simplified　calculation　formula　of　the　longitudinal　fundamental　period　has　been　developed　using　rigidity-based　method.　In　addition,　another　simplified　calculation　formula　was　proposed　based　on　Rayleigh　method,　in　which　the　coupling　of　the　vertical　vibration　of　the　main　girder　and　longitudinal　vibration　of　the　bridge　can　be　accounted　together.　The　accuracy　of　the　proposed　calculating　expressions　was　verified　by　finite　element　analysis　using　SAP2000　with　ten　constructed　cable-stayed　bridges.　It　indicated　that　the　two　formulae　developed　in　this　study,　especially　the　formula　based　on　rigidity-based　method,　were　more　reliable　than　single-mass　model　commonly　used　and　could　be　effectively　applied　in　simplified　calculation　of　the　longitudinal　fundamental　period　of　cable-stayed　bridges.