Abstract: In the intricate marine environment, coastal bridges frequently encounter severe threats posed by extreme waves. Therefore, it is of paramount importance to investigate the dynamic response and failure mechanism of the box-girder superstructure of coastal bridges under the influence of supports and adjacent spans, considering the three-dimensional fluid-structure coupling effect. This research holds significant implications for enhancing the design strategies aimed at preventing and mitigating extreme wave disasters in coastal bridge systems. In this study, a three-dimensional wave-bridge fluid-structure coupling model is established by using LS-DYNA nonlinear finite element program based on the ALE (Arbitrary Lagrange-Euler) method to discretize the Navier-Stokes equations of incompressible fluid. Validation of the accuracy of the three-dimensional fluid-structure coupling model is performed through comparisons with wave flume experiments and a bearing shear modulus test. Subsequently, parametric studies are conducted using this model. A comparison between the three-dimensional coupling model and a two-dimensional coupling model is presented. The effects of different wave parameters and submersion coefficients on the box-girder superstructure are analyzed. And the dynamic response and failure mechanism of the box-girder superstructure under extreme wave action are investigated. The results indicate that, compared with the two-dimensional coupling model, the three-dimensional coupling model can better consider the influence of the three-dimensional effect, the supports and the adjacent span structures on the dynamic characteristics of the box-girder superstructure. With the increase of wave height or the decrease of submersion coefficient, the probability of damage of the box-girder superstructure becomes greater. The failure of the box-girder superstructure under extreme wave action is often accompanied by the shear deformation failure of the bearings. Additionally, issues such as insufficient vertical bearing capacity of the bearings may also arise during the overturning failure of the box-girder superstructure. In the overturning failure of the box-girder superstructure and the shear deformation failure of the bearings, the land side bearings usually suffer large shear deformation and vertical reaction force, and its state plays a dominant role in evaluating whether the shear deformation failure or the vertical bearing capacity failure occurs.