In order to evaluate the bearing capacity of the composite beam bridge with less reinforcement and slightly curved plate during operation and study the reasonable treatment scheme, a 5×16.8 m reinforced concrete I-beam+slightly curved plate composite beam bridge with less reinforcement is taken as the research object. The appearance condition and material condition of the bridge are tested, and the comprehensive evaluation is made by carrying capacity calculation and static load test, and the maintenance and reconstruction scheme is determined by comprehensive comparison. The results show that there are 239 structural cracks in the main girder, and the maximum width of the cracks is 3.14 mm, which far exceeds the limit value of the code. The bearing capacity of the superstructure does not meet the requirements of the code, and the cracks in the micro-bending plate beam still have a trend of continuous expansion. Replacing the upper structure with continuous steel box girder and retaining the lower structure has the advantages of short construction period and low total investment. After transformation, the bridge operates well.