To analyze the applicability of the ultra-high performance concrete (UHPC) reinforcement method for box arch bridges, a reinforcement project of a reinforced concrete box arch bridge with a span of 125 m was taken as the background in this paper. By analyzing the limitations of the steel plate bonding method in reinforcing large-span box arch bridges, a UHPC reinforcement scheme was proposed, and the corresponding simplified calculation formulas were established. The research results indicate that: ① Before reinforcement, the box arch of the bridge exhibits a large eccentric compression failure. After pasting steel plates on the bottom slab of the box arch, the failure mode transforms into a small eccentric compression failure, and the failure location shifts from the tension bottom slab to the compression top slab. However, the bearing capacity requirements still cannot be met, indicating that the section thickness of the compression top slab of the box arch is insufficient; ② When a 6 cm thick UHPC is added to the top and bottom slabs of the box arch, the self-weight of the reinforcement layer is reduced by 60%. The bearing capacities of the sections at the arch foot, 3L/8, and L/2 increase by 68%, 61%, and 48% compared to those before reinforcement, respectively, and the minimum safety factor is 1.05, indicating that the UHPC reinforcement method has a significant reinforcement effect on eccentric compression members such as box arch bridges; ③ The bearing capacity values obtained by the simplified calculation method are in high agreement with the actual bearing capacity values. The maximum error is less than 5%, and the calculated values are slightly smaller than the actual bearing capacity values, indicating that the simplified method has good calculation accuracy and tends to be safe, which can effectively improve the design efficiency.