Abstract: How does the distribution of environmental management powers between the central government and local governments affect the pattern of carbon emissions intensity? Based on the panel data of 284 prefecture-level cities in China from 2005 to 2018, this paper explored the impact of environmental decentralization on urban carbon intensity, and the mediation effect model was used to analyze the mechanism of action. The results show that the impact of environmental decentralization on carbon intensity presented a significant nonlinear “inverted U-shape” characteristic; the mediating effect analysis showed that environmental decentralization promoted carbon intensity by hindering urban environmental governance and technological progress. In addition, the impact of environmental decentralization differed significantly in different cities. Decentralization of environmental management had a significant effect on the carbon intensity of eastern, central and western cities, and the effect was the strongest in western cities; environmental decentralization significantly increased the carbon intensity of large cities and small and medium-sized cities, but had no obvious effect in megacities; environmental decentralization increased the carbon intensity of resource-based and non-resource-based cities, but the effect was not significant in non-resource-based cities. Therefore, in order to realize the positive role of environmental decentralization in reducing carbon intensity, it is necessary to improve the joint management strategy of central-local multi-level governments as soon as possible, promote urban environmental governance and technological progress, and make differentiated division of environmental authority based on the actual situation of the city.