Spatial Effects of Carbon Emission Trading Policy on Energy Efficiency in China: Mediating Role of Financial Efficiency and Industrial Structure

China has implemented Carbon Emission Trading Policy (CETP) to address climate change and the energy crisis since 2013. However, the spatial spillover effects of CETP on energy efficiency have received limited attention. This study addresses this deficiency by applying the spatial difference-in-differences (SDID) model to assess the impact of CETP on energy efficiency and its spillover effects across a balanced panel dataset covering 30 provinces and province-level municipalities of China from 2008 to 2020. The results reveal that CETP has significantly improved regional energy efficiency, with positive spatial spillovers observed across provincial boundaries, implying that CETP's benefits extend beyond geographical borders. Notably, although the spatial correlation of energy efficiency has remained positive, it has shown a gradual weakening trend since 2013. Further mechanistic analysis reveals that CETP improves energy efficiency by optimizing industrial structures and restricting financial efficiency. Finally, the policy has been particularly effective in the western regions of China and in areas with high pollution and stringent environmental law enforcement. This study provides a valuable reference for policymakers on integrating financial and industrial strategies with energy policies and offers a scientific foundation for the potential nationwide expansion of CETP.