Aiming　at　analyzing　the　distinct　coupled　flexural-torsional　vibration　characteristics　of　continuous　and　rigid　frame　bridges　with　thin-walled　box　girder,　a　triple-beam　dynamic　FEM　model　which　can　comprehensively　consider　the　influence　of　free　torsional　stiffness,　restraint　torsional　stiffness　and　rotational　moment　of　inertia　was　presented　based　on　shear-force-flexible　grillage　theory.　The　formulae　for　computing　the　sectional　properties　and　mass　properties　of　the　main　girders　and　the　crossbeams　were　given.　Then　the　dynamic　characteristic　analysis　of　two　given　rigid　frame　bridges　were　carried　out　using　this　model,　the　single　beam　model　and　the　solid　model　respectively　for　comparison.　The　effect　of　the　guard　barrier　was　also　analyzed.　The　results　of　the　presented　model　were　validated　by　that　of　solid　model　and　the　field　test　results.　It　indicates　that　the　triple-beam　model　is　practical　and　reliable　with　high　computational　efficiency.