Under the constraints of the “dual carbon” targets, relaunching the Chinese Certified Emission Reduction Policy (CCER) is essential to supplement and improve China’s carbon market trading. This paper employs interrupted time series analysis and static panel models to assess the impact and mechanism of the CCER policy on carbon emission intensity from 30 provinces in China, comprehensively revealing the policy’s role in promoting the synergy of regional emission reductions. The study finds that: (1) After the implementation of the CCER policy in 2015, carbon emission intensity significantly decreased, while the suspension of the policy in 2017 led to a short-term increase in carbon emission intensity. Despite this rise, the long-term emission reduction trend persisted, though with a diminished effect. Heterogeneity analysis indicates that the policy has a pronounced effect on suppressing carbon emission intensity in the eastern areas with higher technological innovation and non-industrial bases. (2) Higher CCER trading volume can raise carbon emission intensity, but stable carbon market transaction prices help reduce it. The CCER policy helps decrease carbon emission intensity by incentivizing green technological innovation and optimizing energy structure, while it has a negative impact through the scale effect. (3) A significant spatial positive correlation exists in carbon emission intensity’s temporal and spatial distribution. The CCER policy also exerts a restraining influence on neighboring regions through its spatial spillover effect.