Improving carbon emission efficiency (CEE) is a necessary consideration in response to economic downturn and climate change. In this case, industrial symbiosis paves the way for cities to conserve energy, reduce carbon emissions, and upgrade economic development patterns. This paper verifies the influence of industrial symbiosis policies on CEE, represented by National Demonstration Eco-industrial Parks (NDEPs). The difference-in-differences (DID) and spatial DID (SDID) models, as well as panel data of 282 Chinese cities during 2003–2019, were used to complete this argumentation process. The results show that NDEP policy dramatically increases the CEE of pilot cities. Compared with cities without NDEP, pilot cities climb by 3.49% on CEE, mainly due to industrial structure upgrading and green technological innovation. Moreover, eastern, non-resource-based, and multi-NDEP cities experience a noticeable increase in CEE. NDEP increases not only the CEE of pilot cities but also that of their neighboring cities within 450 km through diffusion and demonstration effects. All these findings help promote China’s NDEP construction and offer decision-making guidance for climate governance and low-carbon transition in China and others following a similar pathway.