Abstract: Offshore bridges are facing threats from extreme waves such as tsunamis, which have a severe impact on the substructure of the bridges. The force mechanisms involved in this phenomenon are not yet clear. By using the OpenFOAM open-source software, tsunami simulations based on the Reynolds-averaged Navier-Stokes equations and SST k-ω turbulence model were conducted. Two types of structural models were analyzed and compared: bridge piers without considering the superstructure and bridge piers with superstructure under tsunami wave loads. The horizontal force distribution and force mechanisms of the substructures of offshore box girder bridges under tsunami effects were studied, and the influence of different force distributions on pier moments was analyzed. The research results show that after considering the influence of the box girders on the fluid, the peak horizontal force exerted by the tsunami on the bridge piers in the bridge pier-superstructure model is 1.45 to 1.96 times that of the standalone pier model. The force distribution on the bridge piers in the bridge pier-superstructure model varies significantly, with the maximum force at the top of the pier gradually decreasing downwards. In contrast, the force distribution on the standalone pier model due to the tsunami wave loads is relatively uniform, with little variation from top to bottom. The peak torque value of the tsunami acting on the bridge pier in the pier-beam model is between 1.60-2.33 times that of the isolated pier model. When assessing the structural response of substructures of offshore box girder bridges under tsunami effects, the influence of the superstructure on the surrounding fluid should be fully considered. This research is of significant importance for determining the failure modes of bridge piers and formulating disaster mitigation and protection measures due to the threat from extreme waves of their near tsunamis.