A　method　of　analysis　is　presented　for　the　free　vibration　of　straight　suspension　bridges　with　a　long　span,　stiffened　by　plate　girder　and　box　beam　(or　truss).　The　method　takes　combining　spline　finite　element　with　spline　finite　strip　to　investigate　the　vertical　and　torsional　vibration　as　well　as　their　coupling.　It　uses　variational　principle　to　obtain　dynamic　characteristics　matrices　for　solving　the　eigenproblem.　On　this　basis,　a　spline　model　of　the　whole　bridge　corresponding　to　the　tunnel　test　is　established　for　flutter　analysis.　The　spline　expression　of　aerodynamic　force　on　flutter　is　derived　and　the　fininal　eigenvalue　solution　will　produce　the　desired　flutter　frequency　and　critical　velocity　of　wind.　Numerical　examples　are　shown　to　demonstrate　the　applicability　of　the　method.