The Southeast Asian steel industry stands at a critical historical juncture. On one hand, the region is experiencing an unprecedented wave of capacity expansion, with Chinese capital-driven investments planning to add over 100 million mt of new capacity, and a duopoly between Vietnam and Indonesia has already taken shape. On the other hand, the EU carbon border tax and global decarbonization requirements are compelling industry to undergo a green transformation, posing severe challenges to traditional development models. Key trends indicate that a capacity race and green transformation are advancing in tandem; regional supply chains are being restructured, with self-sufficiency rates increasing, yet high-end products still rely on imports; national infrastructure projects provide solid support for demand, but the risk of overcapacity cannot be ignored. The key to future competition lies in balancing scale expansion with quality improvement, traditional capacity with green transformation, and short-term gains with long-term sustainability. Enterprises that can achieve breakthroughs in low-carbon technologies, product structures, and market positioning will take the lead in the next round of industrial reshuffling. The major transformation in Southeast Asia's steel industry is not only reshaping the regional industrial landscape but also profoundly impact global steel trade and supply chains.