<p>How carbon emissions trading systems (ETS) reshape regional economic and environmental outcomes through capital mobility remains poorly understood, particularly under conditions of asymmetric policy implementation. This study takes China’s pilot carbon markets as a quasi-natural experiment to investigate how the ETS affects low-carbon economic efficiency through the mediating channels of bidirectional capital flows and interregional competition. Using a difference-in-differences framework and spatial econometric techniques, we find that the ETS significantly improves low-carbon economic efficiency in pilot regions while generating negative spillover effects on neighboring non-pilot regions. Mechanism analysis reveals a dual capital-flow channel: inward foreign direct investment enhances domestic efficiency through technology and management spillovers (the pollution halo effect), whereas outward investment by domestic firms induces the relocation of carbon-intensive activities abroad, alleviating local carbon constraints but amplifying carbon leakage risks. These findings suggest that ETS outcomes are shaped not only by innovation incentives but also by strategic capital reallocation and regional competition under regulatory asymmetry. By integrating institutional design, capital mobility, and spatial interaction into a unified analytical framework, this study moves the ETS literature beyond isolated innovation or leakage perspectives. The results further provide policy insights for preventing carbon leakage, guiding green capital flows, and strengthening coordination in the development of a unified national carbon market.</p>