The　simplified　calculation　of　the　first-order　longitudinal　vibration　period　for　a　cable-stayed　bridge　is　very　important　for　the　comparison　of　design　plans　and　the　evaluation　of　seismic　performance.　Firstly,　according　to　the　longitudinal　seismic　inertia　force　transmission　of　cable-stayed　bridges,　the　double-mass　model　derived　by　flexibility　method　was　developed　to　simplify　the　calculation　of　the　first-order　longitudinal　vibration.　Based　on　significant　coupling　between　the　longitudinal　modes　and　vertical　modes,　the　simplified　calculation　of　the　first-order　longitudinal　vibration　period　was　then　investigated　by　energy　principle　in　fixed　hinge　cable-stayed　bridges.　Finally,　the　two　formulas　were　evaluated　by　the　tests　on　ten　built-up　bridges.　It　is　concluded　that　these　two　simplified　formulas　were　in　good　agreement　with　those　predicated　by　finite　element　method.　The　proposed　double-mass　model　has　higher　accuracy　and　reliability.　©　2017,　Editorial　Department　of　Journal　of　Hunan　University.　All　right　reserved.