With　the　development　of　the　economy　and　transportation,　more　and　more　cross-sea　bridges　are　appearing　in　people＇s　view.　However,　cross-sea　bridges　are　not　only　subjected　to　repeated　traffic　loads　but　also　to　corrosion　from　the　marine　environment.　In　addition,　the　steel　box　girder　deck　plate　is　directly　subjected　to　repeated　vehicle　loads,　and　its　welded　details　are　highly　susceptible　to　fatigue　damage　and　destruction,　resulting　in　fatigue　or　corrosion　damage　to　the　welded　top　plate-U　rib　of　the　cross-sea　steel　box　girder.　This　paper　is　based　on　fracture　mechanics　analysis　methods　and　finite　element　simulation　analysis,　in　order　to　establish　the　numerical　analysis　method　of　the　welded　top　plate-U　rib　of　the　steel　box　girder　deck　plate　fatigue,　which　is　to　take　into　account　the　coupling　of　the　marine　corrosive　environment　and　fatigue　cyclic　loading,　explore　the　welded　top　plate-U　rib　of　the　stress　intensity　factor,　crack　expansion,　crack　expansion　life　and　total　fatigue　life　of　the　welded　details.　Furthermore,　the　establishment　of　the　welded　top　plate-U　rib　corrosion　fatigue　life　assessment　model　is　to　explore　the　fatigue　life　of　different　top　plate　thicknesses.　The　results　show　that:　the　stress　intensity　factor　along　the　crack　front　is　symmetrically　distributed,　and　with　the　increase　in　the　crack　expansion　step,　the　corresponding　stress　intensity　factor　amplitude　is　also　increasing;　in　addition,　with　the　increase　in　the　top　plate　thickness,　fatigue　crack　expansion　rate　significantly　reduced.