<p>Bridge pier is the essential element in beam bridge structure, playing a crucial role in overall performance. The accuracy and feasibility of the mechanical model significantly influence the prediction of structural behavior, making it necessary to develop a complete equivalent mechanical model and rapid damage assessment method for bridge piers to ensure the safety and stability of the bridge. An improved mechanical model for the bridge pier is presented according to mechanical constraints and component composition patterns, which accounts for bending-shear deformation and boundary conditions. For continuous beam bridge piers, the stiffness matrix of the equivalent model is derived using the flexibility method combined with modified Euler–Bernoulli beam theory. Rotational degrees of freedom are eliminated through static–dynamic condensation, and unknown mass and stiffness parameters are identified using the extended Kalman filter method, resulting in a complete equivalent mechanical model. A damage assessment process is then proposed based on the extended Kalman filter method, simulating the finite element model of the continuous beam bridge. In addition, damage quantification of the reinforced concrete pier is carried out under incremental loading conditions. The results indicate that the stiffness degradation rates of certain elements in the finite element model closely match the theoretical predictions. Moreover, the observed trends in structural stiffness from the pier test coincide with the experimental results. In general, the proposed equivalent mechanical model and damage assessment method demonstrate significant applicability in engineering practice.</p>