Urban infrastructures are facing environmental degradation induced by surging energy demand. There is an urgent need for countries to create a greener and more efficient infrastructure. Smart infrastructure (SI), as a new type of digitally enhanced infrastructure, has a strong potential to reduce carbon emissions. This study examines the nexus between smart infrastructure and carbon emission intensity (CI) in China, encompassing 251 cities from 2011 to 2019. The findings indicate that: (1) Every 1 % improvement in smart infrastructure reduces carbon emission intensity by 0.2810 %. This effect is significant only in more advantageous geographic locations and non-resource-dependent cities. (2) Panel threshold regression analysis demonstrates a nonlinear relationship between smart infrastructure and carbon intensity, showing a decreasing trend followed by an increasing trend under different energy consumption levels. (3) Mechanism analyses show that smart infrastructure affects carbon intensity through technological innovation and agglomeration effects. (4) Smart infrastructure exerts spatial spillover effects on carbon intensity with substantial spatial decay effects. This paper deepens the understanding of smart infrastructures and provides an empirical basis for the deployment of smart infrastructures to achieve carbon neutrality.