To address the urgent need for post-earthquake structural performance assessment of existing bridges, a rapid seismic performance assessment method integrating multi-source monitoring data has been proposed. A 4.7-magnitude earthquake in Feidong County, Hefei City, was selected as the case study, and a multi-level performance assessment framework covering energy transmission analysis, displacement stability evaluation, and seismic damage index estimation was established based on synchronous ground acceleration time histories and structural monitoring data, including main girder strain and bearing displacement. The seismic performance of the main girder and the stability of the beam ends were assessed by quantifying seismic input, energy transfer, and structural response characteristics based on energy transfer theory. On this basis, the seismic damage index of the substructure was quantitatively assessed by developing a vulnerability matrix from ground-motion acceleration and pier-top displacement data and by deriving an exceedance probability model using the Beta distribution. The results show that the energy transfer ratios of the main girder are all below 0.005. The vibration response is weak, and no energy concentration or amplification effects are observed. The longitudinal displacement variation of the bearings before and after the earthquake is significantly smaller than the theoretical allowable value, indicating that the boundary conditions remain stable. The seismic damage index of the pier system is approximately 0.39, which indicates slight damage and remains below the performance degradation threshold. The proposed method enables rapid post-earthquake performance assessment of existing bridges and provides technical support for post-earthquake damage diagnosis and functional recovery decision-making for similar bridges.