To address the insufficient correlation between static and dynamic load test data and the difficulty in quantifying damage in existing bridge evaluations, a 28-year-old prestressed concrete girder bridge in Hunan Province was studied. Through field static and dynamic load tests combined with finite element simulation, the bearing capacity of the bridge was systematically evaluated, with emphasis on the impact of deck deterioration on structural performance. The results show that under static load, the deflection calibration coefficients range from 0.55 to 0.65 and strain calibration coefficients from 0.64 to 0.83, indicating that the overall stiffness meets design requirements. In dynamic load tests, the maximum vehicle impact coefficient during jump tests reaches 0.32 (average 0.26), both exceeding the design value of 0.25, quantitatively revealing the significant amplification effect of deck pavement damage on vehicle impact. Following comprehensive analysis and in accordance with the Specifications for Maintenance of Highway Bridges and Culverts (JTG 5120-2021), the bridge is classified as Category 3, recommending localized repair of the deck pavement. This study provides a methodological reference for the bearing capacity evaluation and maintenance decision-making of similar bridges.