To address the limitations of bridge traditional monolithic retaining blocks in the seismic reinforcement and post-earthquake repair, this study proposed an innovative externally prestressed prefabricated ultra-high performance concrete (UHPC) retaining block and investigated systematically its seismic performance. Initially, the study initially examined the construction procedures and structural advantages of the proposed retaining block, providing relevant practical construction suggestions to ensure its engineering feasibility. Subsequently, based on a validated ABAQUS model, further simulations are conducted to comparatively analyze the seismic response of the bridge under three retaining block conditions: no block, monolithic block, and prefabricated block. The Results indicated that the retaining block could effectively limit the lateral displacement of the girder and bearing, thereby playing a crucial role in preventing girder drop and protecting the bearing. However, the collision between the retaining block and the girder enhanced the seismic response of the piers and other substructures in bridge, increasing their damage risk. Therefore, the design of the retaining block should balance the bearing capacity with displacement performance. The externally prestressed prefabricated UHPC retaining block exhibited seismic performance comparable to that of the monolithic block, with less self-damage. This prefabricated retaining block structure, characterized by its simple operation and controllable performance, provided a new approach for the seismic reinforcement and post-earthquake repair of bridge retaining blocks.