Weathering steel is suitable for bridge construction in harsh environments, such as high-altitude cold regions. However, the corrosion resistance mechanism of weathering steel in these areas remains unclear. To investigate the weathering mechanism of bridge weathering steel in high-altitude cold regions, this study focuses on commonly used bridge weathering steel and ordinary steel. An in-situ corrosion test of bridge steel was conducted in a typical high-altitude cold environment. By analyzing corrosion weight loss data, the macroscopic appearance of the rust layer, the microscopic morphology of the rust layer, and the elemental distribution along the rust layer, the corrosion resistance mechanism of weathering steel in high-altitude cold regions was examined. The results show that after a 24-month corrosion period, the corrosion thickness loss of weathering steel is approximately 10% lower than that of ordinary carbon steel. The formation of goethite in the rust products of weathering steel hinders the penetration of oxygen, moisture, and other elements into the rust layer, which is the primary reason for its weather resistance in high-altitude cold regions. During the corrosion process, chromium (Cr) accumulates in the inner rust layer of the weathering steel, causing the inner rust layer to become denser in subsequent corrosion, further enhancing corrosion resistance. The findings of this study provide theoretical and data support for bridge engineering construction in high-altitude cold regions.