Built in 1964, Nanning Bridge is one of the earliest cantilever reinforced concrete thin-walled box urban Bridges in China. Due to the limitation of the design concept at that time and the influence of the increasing traffic volume, the design load grade of the bridge is low and can not meet the requirements of the current load. In addition, after 45 years of operation, various diseases have emerged. OVM unbonded external prestressing system is adopted to reduce the secondary damage to the original structure in the process of reinforcement. In the design, the structure of the tooth plate is optimized to make the force of the tooth plate in a good state. At the same time, the construction method is optimized to make the connection between the new tooth plate and the original structure reliable; In the tensioning process, the "suspension" tensioning method is used to effectively prevent the damage to the working clip during the tensioning process. By adopting the above main strengthening measures such as unbonded external prestressing, the structural force is improved.