This study investigated the design scheme for reinforcing the loaded steel truss bridge using the section enlargement method through the Songpu Bridge rehabilitation project. It clarified the internal force calculation method and stress control principle for both original members and reinforced components and analyzed the influence of initial stresses in original members on the reinforcement effect. A combined theoretical and experimental approach was adopted, and an internal force distribution calculation method that considered the staged load-bearing process of reinforced components and original members was proposed, with the edge yield criterion of original members established as the stress control principle. Comparative analyses against the calculation methods specified in the Standard for Design of Strengthening Steel Structures (GB 51367?2019) and bridge completion test data validated the rationality and effectiveness of the proposed reinforcement design scheme. The results demonstrate that the developed internal force distribution calculation method accurately calculates the internal force of members, while the edge yield criterion of original members ensures the reinforced steel bridge satisfies elastic stress requirements. Additionally, the study reveals that initial stress levels in original members significantly affect the reinforcement effect. It is recommended to implement partial unloading before reinforcement and control initial stress levels below 0.6.