Expansion joints are a crucial component of bridges, influencing not only the force state, service performance, and operational safety of the structures but also playing a substantial role in ensuring the comfort and safety of traffic on the bridge deck. This paper investigates the static mechanical responses of bridge modular expansion joints using a full-scale 3D solid finite element (FE) analysis method. Firstly, the working mechanism, force transmission path, and influencing factors of the modular expansion joint are analyzed. Secondly, a full-size 3D solid finite model of the modular expansion joint is established using ANSYS FE software and APDL parametric modeling. The model incorporates the actual structure, force transmission path, contact effects, and boundary conditions of the expansion joint. Then, the load size and action position of typical vehicles on the joint are analyzed to obtain adverse vehicle action. Finally, the force state of the modular expansion joint under vehicle load using the 3D solid FE model is calculated and discussed. This paper proposes a precise FE analysis method for the complex stress state calculation of bridge modular expansion joints, providing theoretical and technical support for damage identification, state evaluation, and safety assessment of such joints, and can also be learned and used for other types of bridge expansion joints.