This article examines the evolution of China-ASEAN environmental and energy cooperation since the 2010s with a focus on changes following the launch of the Belt and Road Initiative (BRI). It shows that over the past decade China and ASEAN have actively expanded cooperative frameworks and mechanisms in the domain of environmental protection as part of their comprehensive approach to sustainable development, particularly regarding energy projects. Nonetheless, the implementation record in their energy cooperation, that is, the lion's share of BRI projects in Southeast Asia, remains unsustainable and contradicts with their shared "green" policy discourse. The article examines factors from both sides of China and ASEAN that contribute to this disconnect and explains why China's own accomplishments in renewable energy development fail to spill over to Southeast Asia. These findings have implications for China's impact on global governance and its leadership role in the Global South.