Informal environmental regulation, represented by public participation, has an increasingly significant role in environmental governance. This paper utilizes panel data of 285 cities in China from 2003 to 2017. It examines the difference-in-differences (DID) and instrumental variable method (IV) to investigate the causal effect of public participation represented by Environmental Non- Governmental Organizations(ENGOs) on regional carbon emissions. The empirical results show that public participation reduces regional carbon emissions, which still holds after a series of endogeneity and robustness tests. This paper proves an inverted U-shaped nonlinear relationship between the intensity of public participation and regional carbon emissions. Furthermore, this paper demonstrates that regional green technology innovation and strengthening formal environmental regulations are the primary mechanisms for public involvement in promoting regional carbon emission reduction. Finally, this paper discusses the heterogeneity governance effect among cities and finds that the governance effect of the sample is more pronounced in eastern cities, non-resource-based cities, large cities, and provincial capitals. The results reveal the importance of public participation in regional carbon emission reduction and provide an empirical basis for promoting informal environmental regulation.