Offshore　bridges　need　to　bear　complex　wave　and　current　loads　and　face　the　threat　of　extreme　loads　such　as　tsunamis.　Potential　tsunamis　will　have　disastrous　effects　on　coastal　areas.　Compared　with　conventional　beam　bridges,　there　are　few　studies　on　rigid　frame　box　girder　bridges.　In　this　paper,　based　on　a　continuous　rigid　frame　box　girder　bridge　in　the　offshore　area,　the　parameters　of　tsunami　impact　force　are　analyzed.　First,　based　on　Stoke　theory　and　water　level　in　the　storage　area,　the　water　level　in　the　static　water　area　and　quasi　stable　area　in　the　dam　break　wave　working　conditions　are　obtained,　and　12　kinds　of　dam　break　wave　working　conditions　are　preliminarily　proposed　to　be　used　for　the　background　bridge.　Then,　according　to　the　determined　simulation　working　conditions,　the　box　girder　dam　break　impact　numerical　model　is　simulated.　According　to　the　calculated　results,　the　stress　characteristics　of　the　box　girder　under　the　tsunami　impact　are　described　in　the　way　of　time　history　stage　division,　and　on　this　basis,　The　stress　mechanism　is　analyzed　from　the　whole　level　and　local　level.　The　results　show　that　on　the　whole　level,　the　variation　of　horizontal　force　and　vertical　force　with　time　can　correspond　to　the　variation　of　dam　break　flow　around　the　box　girder.　At　the　local　level,　the　horizontal　wall　boundary　bears　the　vertical　force,　the　vertical　wall　boundary　bears　the　horizontal　force,　and　the　inclined　wall　boundary　bears　both　the　horizontal　force　and　the　vertical　force.　In　addition,　negative　pressure　is　the　cause　of　suction　on　the　wall　boundary.　It　provides　a　reference　for　the　calculation　and　analysis　of　tsunami　load　of　offshore　rigid　frame　box　girder　bridges,　the　identification　of　bridge　failure　modes　and　the　formulation　of　protective　measures.　　©　Published　under　licence　by　IOP　Publishing　Ltd.