Environmental factors significantly affect the fatigue performance of weathering steel welded components in high-altitude, low-temperature corrosive environments. This study conducted multi-factor-coupled constant-amplitude fatigue tests on Q500qENH weathering steel V-groove welded joints and built an equivalent finite element model using test data to explore key influencing factors under multi-condition coupling. Results show that stress level most significantly affects fatigue performance, followed by corrosion duration, then ambient temperature, with influences decreasing in turn. Analyzing 18-day cyclic immersion corrosion morphology predicts 21-year outdoor corrosion in plateau regions, providing a reliable method for long-term exposure prediction. Finite element simulations confirm that low temperatures improve slightly corroded specimens’ fatigue performance by 20%, but damage accumulates before optimal service. This study offers key parameters for safe design of high-altitude weathering steel welded components.

In high-altitude corrosive environments, weathering steel is widely applied due to its excellent corrosion resistance. However, the welded joint regions, where the chemical composition and microstructure undergo changes, are susceptible to the corrosion-induced degradation of mechanical properties. This study investigates the corrosion–mechanical synergistic degradation behavior of a 16 mm thick Q500 qENH base metal and its V-type and Y-type welded joint specimens. Periodic immersion corrosion tests were conducted to simulate plateau atmospheric conditions, followed by mechanical performance evaluations. Corrosion metrics—including corrosion rate, cross-sectional loss, penetration depth, and corrosion progression speed—were analyzed in relation to mechanical indicators such as the fracture location, yield load, ultimate load, yield strength, and tensile strength at varying exposure durations. The results indicate that the corrosion process exhibits distinct layering, with a two-stage characteristic of rapid initial corrosion followed by slower progression. Welded joints consistently exhibit higher corrosion rates than the base metal, with the rate difference evolving nonlinearly in an “increase–decrease–stabilization” trend. After corrosion, the mechanical performance degradation of welded joint specimens is more severe than that of base metal specimens.