In order to solve the problem of beam cracking during the construction of an urban elevated concrete cast-in-situ widening box beam bridge in soft soil foundation, combined with the characteristics of crack distribution, the corresponding disposal measures are proposed, and its reinforcement effect is analyzed by the method of combining the space rod system model and the ANSYS solid finite element model. The results show that the anti-bending bearing capacity of of the positive section of the beam is increased by 9.0%, and the shear bearing capacity of the oblique section is increased by 38.9% compared with that before the reinforcement. The bending resistance of positive section, the shear resistance of the oblique section and the direct shear bearing capacity of the crack section all meet the requirements after the reinforcement of the beam. And the reinforcement effect is more significant. The load test results of the reinforced beam show that the overall bending performance and the shear performance near the cracking position of the reinforced beam are good. The enlarged section and additional prestressed comprehensive reinforcement scheme can provide reference for other projects.