Abstract: The traditional reinforcement method for long-span steel box girders has disadvantages， such as poor recovery of initial deflection deformation and insufficient bearing capacity. In order to solve these problems， a construction method for strengthening existing long-span steel box girders using the string beam stress system is proposed in this paper. Specifically， the method involves arranging the vertical supports reasonably beneath the steel box girder according to a certain distance， and connecting them to the steel box girder using prestressed cables， thereby forming a string beam stress system. The working principle of the reinforcement system is to generate a negative bending moment on the steel box girder through the pre-tension of the cable， so as to realize the structural deflection deformation recovery and the bearing capacity improvement. This self-balancing feature ensures that the steel box girder does not produce horizontal forces during the reinforcement process. It minimizes little impact on the original support， eliminates the require for special adjustments， and improves the convenience and safety of construction. Using MIDAS Gen， an analysis model of the strengthened structure with a string beam was established. The construction steps were set reasonably， so that the strengthening effect of the string beam began to appear after the steel box beam had produced a certain deflection. The inverse iteration method was used to determine the shape of the string beam structure and optimize the cable pretension. The structural stresses and deflections of steel box girders under different working conditions were calculated， and the changes of structural bearing capacity as well as reinforcement effects were analyzed. In addition， the calculation models of beam elements and shell elements were respectively employed in the finite element simulation analysis process， and a comparative analysis was conducted on the differences in bearing capacity and deformation between the two models for the string beam-reinforced structure. The simulation results showed a linear relation between cable pretension and structure deflection. When the cable pretension was 4000 kN， the deflection of the structure under the set working condition was 0 mm； under the full load condition， the deflection of the beam-reinforced structure system was 63.39 mm， and the maximum stress was 227.9 MPa， both of which met the specification requirements of GB 50017—2017. Compared with the beam element model， the deflection calculation results of the shell element model were smaller， and the stress calculation results were larger. The main reason was that the shell element model provided a more comprehensive consideration of the overall stiffness of the structure， and the mesh refinement accurately reflected the local stress situation in the structure. After conducting structural finite element analysis， the section design， optimization， and local stability analysis of structural members were further analyzed to ensure the safety and economy of the structure. The comprehensive research results showed that reinforcing the existing long-span steel box girders using the string beam stress system could effectively restore the initial deformation of the structure， improve its bearing capacity， and ensure construction safety and convenience.