Weathering steel is suitable for bridge construction in harsh environmental areas such as high-altitude cold areas, but the current corrosion resistance mechanism of weathering steel in high-altitude cold areas is unclear. To reveal it, typical bridge weathering steel and ordinary steel were used as research objects. In-situ corrosion tests of bridge steel were carried out in typical high-altitude cold areas to explore the feasibility of applying weathering steel in high-altitude cold areas. Firstly, 20 in-situ corrosion specimens were designed according to the specifications, and in-situ corrosion tests with corrosion cycles of 2, 4, 10, 18 and 24 months were carried out; combined with corrosion weight loss data, rust layer macromorphology and micromorphology, the distribution of elements along the rust layer, the corrosion resistance mechanism of weathering steel in high-altitude cold areas was analyzed. The research results show that the corrosion loss of the two specimens in Kangding atmospheric environment is basically the same before the corrosion cycle is 10 months; when the corrosion cycle is 24 months, the corrosion loss of weathering steel is about 10% lower than that of ordinary carbon steel; the goethite formed in the rust products of weathering steel hinders the accumulation of oxygen, moisture, etc. in the rust layer, further preventing its corrosion; during the corrosion process, Cr is enriched in the inner rust layer of weathering steel, causing the inner rust layer to become denser in subsequent corrosion, further enhancing the corrosion resistance.