Abstract: In high-altitude corrosive environments， weathering steel is extensively utilized due to its superior corrosion resistance. However， welded joint regions， characterized by altered chemical compositions and microstructures， are prone to attack by corrosive media， leading to mechanical property degradation. This study investigates 16 mm-thick Q500qENH base metal and V-shaped/Y-shaped welded joint specimens using a cyclic immersion corrosion test designed to replicate the realistic dry-wet alternating conditions of plateau environments. Monotonic tensile tests were subsequently performed on specimens subjected to varying corrosion durations. Through comprehensive analysis of fracture locations， yield load， ultimate load， yield strength， and tensile strength， the performance degradation mechanism of weathering steel welded joints under typical high-altitude corrosion was elucidated. The results demonstrated that welding reduced corrosion resistance and enhanced brittleness， evidenced by greater mechanical property degradation in welded joints compared to base metal specimens. For the base metal， its strength degraded rapidly in the early stage of corrosion， while the degradation rate slowed down in the later stage due to the densification of the rust layer. Among the welded joints， Y-shaped joints were the most sensitive to corrosion but had the highest initial strength. Most specimens exhibited ductile failure， and only some V-shaped joints showed brittle fracture after 27 days of corrosion.In this study， a predictive model for the yield strength of the base metal （applicable to an average corrosion rate range of 0~3%） was established. Additionally， a strength reduction factor of 0.97 （corresponding to a corrosion rate of 2.30%） was proposed as the maintenance threshold. This research provides a reference for the engineering design of weathering steel structures in plateau environments.