Based on the reinforcement project of Nandu Bridge, the paper analyzes the main diseases (including concrete carbonization, steel rust expansion, concrete defects, etc.) affecting the reduction of the bearing capacity of the main arch circle and their causes. In order to improve the bearing capacity of the original main arch circle to meet the current traffic needs, the strengthening technology of the main arch circle is adopted to increase the cross-section of the main arch circle To provide reference for similar engineering design and construction.