Abstract:The weathering steel bridge has the advantages of superior fatigue and mechanical properties, strong corrosion resistance, fast factory processing and manufacturing speed, small on-site construction interference, low maintenance and maintenance costs in the late period, and has been built in large quantities abroad. But in China, there are few construction projects and research results of weathering steel bridge. In order to explore the design and application of small and medium-sized span weatherproof steel box composite beams in the southwest mountainous area, taking Lijiagou Bridge in National Highway 212 as the engineering background, absorbing the experience in the construction and development of weatherproof steel bridges at home and abroad, and aiming at the special construction environment in the southwest mountainous area, the overall design, steel selection, structural layout, structural details design and later maintenance of the non-painting weathering steel box composite beam bridges are studied, which provides a reference for the construction of weathering steel bridges under the similar conditions.