Low-carbon city pilot policy (LCCP) is an innovative initiative for promoting low-carbon transformation and green development in China, which is of great practical significance for realizing China's vision of “double carbon” on schedule. In this study, LCCP implementation is treated as a “quasi-natural experiment,” and the spatial difference-in-differences approach is used to quantitatively examine the carbon reduction effects and impact mechanisms of LCCP using panel data of 283 cities in China from 2006 to 2017. The results show that since 2011, LCCP has significantly reduced the carbon intensity of the pilot cities by 0.13%, resulting from the effects of urban environmental governance, industrial structure optimization, and urban innovation level improvement. Meanwhile, there is a significant spatial spillover effect which results in a 0.9% reduction in carbon intensity of neighboring cities. The spatial spillover range of the reduction effect is about 500 km, which decays with distance. Moreover, the carbon reduction effect of the policy is spatially and temporally heterogeneous, and the reduction effect is more significant for resource-based cities, and different for resource-based cities in different development stages. The above findings provide useful policy insights for constructing low-carbon cities under China's “dual carbon goals” and help to realize the win–win path of green development and carbon reduction transition.