Subsea well tubular strings in the South China Sea are chronically exposed to high-temperature CO₂ corrosive environments, where the synergistic effects of elevated temperature and corrosion significantly elevate the risk of integrity failure, posing a critical challenge to the safe and efficient development of deepwater reservoirs. To address tubular corrosion-induced failure in deepwater high-temperature gas wells under CO₂ environments, the effects of high-temperature-induced strength degradation and corrosion damage under extreme service conditions were incorporated. A safety evaluation methodology for tubular strings was developed by coupling thermomechanical and corrosion damage mechanisms. Furthermore, an optimized tubular design strategy based on composite mitigation measures was proposed, systematically evaluating the influence mechanisms of different mitigation combinations on tubular performance, followed by case study validation. The results demonstrate that thermal and corrosive interactions lead to progressive tubular degradation. During the initial service phase, corrosion is negligible, and strength deterioration due to high temperature dominates. However, as service duration increases, accelerated corrosion causes rapid strength attenuation. Under severe corrosion conditions, corrosion inhibitors and corrosion-resistant alloys significantly reduce corrosion rates and enhance safety, whereas increasing tubular steel grade provides limited improvement. A hybrid segmented tubular design incorporating 80-ksi carbon steel + 110-ksi carbon steel + 80-ksi corrosion-resistant alloys with corrosion inhibitors injection was shown to optimize structural integrity while minimizing operational costs.