Abstract: Under the fiscal decentralization system,environmental regulation has become a strategic means of competition for mobile resources between regions. Cross-regional environmental regulation,however,lacks mandatory binding capabilities,and its utility and operational mechanisms remain to be further validated. This study focused on the regional coordinated development policy as a form of cross-regional environmental regulation. Utilizing panel data from 273 prefecture-level cities in China from 2007 to 2019,a two-way fixed-effects model with propensity score matching was employed for a difference-in-differences analysis. The empirical examination was used to investigate the impact and mechanisms of the regional coordinated development policy on carbon emissions. The results indicated that the policy significantly promoted the reduction of carbon emissions in cities,and this conclusion was valid after a series of robustness tests. Mechanism analysis suggested that the effects of regional coordinated development policy be achieved by encouraging cooperation and coordination among different cities,optimizing resource allocation,facilitating industrial upgrading,increasing investment in technological elements,promoting the research and application of low-carbon technologies,and improving energy efficiency to reduce carbon emissions. Heterogeneous analysis revealed that the policy effects varied with the heterogeneity of pilot cities in terms of population size and fiscal levels. Larger cities with greater population sizes demonstrated stronger competitive advantages in the development of low-carbon industries and the application of green technologies. Cities with higher fiscal expenditures,on the other hand,promoted the development of low-carbon industries by nurturing and introducing advanced low-carbon technologies,providing financial support and incentive measures,and allocating more funds to infrastructure construction. These enhanced the effectiveness of regional coordinated development policy in reducing carbon emissions.