Based on the panel data of 267 prefecture-level cities in China from 2003 to 2016, this paper adopts propensity score matching (PSM) and difference-in-difference (DID) as research methods to test and analyze the impact effect of the county-to-district reform on the environmental pollution. The results show that: (1) The county-to-district reforms have significantly increased the urban environmental pollution. After changing the time and space sample size of the reformed cities, there is no obvious difference in the estimated results; (2) In terms of time, the impact of the county-to-district reforms on environmental pollution has a short-term dynamic, and there is a difference between industrial wastewater pollution and industrial waste-gas pollution; (3) By region, the eastern cities have significantly increased the level of environmental pollution after the county-to-district reforms, both the coefficient and the significance level of the cities in the Mid-West are weaker than those in the East, and presents Eastern > Central > Western; (4) Mechanism testing shows that the county-to-district reforms significantly expand urban space and agglomerate population. The former exacerbates the effects of environmental pollution, while the latter suppresses the growth of environmental pollution. Therefore, it is necessary for the government to reduce the institutional constraints of population migration to big cities and blind land expansion so as to promote pollution reduction.

County-to-district reform (CTDR) is an important policy path for the government to promote the cultivation and construction of urban agglomerations, and exploring its “carbon emission” effect is of great significance for the high-quality development of urban agglomerations and the realization of the “dual carbon” goal. Based on the panel data of 120 counties in the Yangtze River Delta urban agglomeration from 2000–2017, this paper empirically tests the effect of county-to-district reforms on per capita carbon emissions in the counties of the central and peripheral cities of the Yangtze River Delta urban agglomeration under the Kutznets curve (EKC) hypothesis and the integrated difference-in-difference (DID) model and STIRPAT model. The results show that: (1) The carbon emission effect of county-to-district reforms have significant regional heterogeneity. The reforms of the central city of the urban agglomeration significantly reduced the per capita carbon emission of the county by 4.27%, whereas the reforms of the periphery cities of the urban agglomeration significantly increased per capita carbon emission by 6.56%. (2) The impact of county-to-district reforms on county per capita carbon emissions began to appear in the fourth year of reform. (3) Mechanism analysis showed that county-to-district reforms promoted central cities population agglomeration and reduction of carbon emission intensity can help reduce the per capita carbon emission level in counties, whereas peripheral cities have a dual carbon-increasing effect of decreasing population density and increasing carbon emission intensity. Therefore, the approval of county-to-district reforms should be strictly controlled, and the reform of non-central cities would be especially prudent, so as to reduce the negative effect of reform on the high-quality development of cities.