At present, in most of the studies on the application of floating breakwaters to the protection of bridge structures, the study of mooring floating breakwaters to the protection of coastal Bridges under extreme waves is still limited. Based on the coupling of the waves2foam tool in the open-source computational fluid dynamics(CFD) software OpenFOAM and the open-source MoorDyn mooring tool, the hydrodynamic model of the floating breakwater and box girder under the action of isolated waves was established under extreme conditions. The effects of submerged coefficient of floating breakwater, length of floating breakwater and length of mooring mop on the protective characteristics of box girder were studied, and the pressure of flow field and characteristic points of box girder were analyzed to reveal the reason for the change of force characteristics of box girder. The results show that the coupling method used has high accuracy and reliability. Floating breakwaters have a significant contribution to reducing horizontal loads and a small contribution to reducing vertical loads. It is necessary to enhance the vertical impact resistance constraint of the box girder. The buoyancy force has a significant impact on the peak pressure of characteristic points. Increasing the submergence coefficient of the breakwater appropriately has a better comprehensive protection effect on the box girder.