In order to accurately assess the actual corrosion damage of weathering steel bridges under service conditions, accelerated corrosion tests were conducted on Q355NHB and Q355B steels under four stress ratios and five exposure durations. Microscopic observation and surface morphology scanning were employed to investigate the evolution of corrosion morphology, as well as the temporal and stress-dependent development of surface roughness and mass loss rate. The results show that the influence of stress is most pronounced during the early stages of corrosion, with pits progressively coalescing into stepped morphologies exceeding 2 000 μm in diameter as corrosion duration increased. An increase in stress ratio is found to induce grain refinement, with the maximum reduction in Feret diameter reaching 31.92% relative to the stress-free condition. Moreover, surface roughness and mass loss rate under stress conditions are elevated by 30% and 6.63%, respectively, compared to those observed in the absence of stress. It is concluded that omitting the effects of mechanical stress in corrosion assessments may lead to a substantial underestimation of actual material degradation in structural steels.