Abstract: Under the goal of “double carbon”, how to promote high-quality economic development is a practical problem that China needs to be solved, and industrial co-agglomeration is one of the feasible paths to achieve low-carbon development. The article has incorporated carbon intensity, industrial co-agglomeration and industrial structure upgrading into a comprehensive analysis framework and uses data from 283 prefecture-level and above cities in China from 2008 to 2017 to empirically test the impact of industrial co-agglomeration between secondary and tertiary industries on carbon intensity and the path mechanism using dynamic spatial Durbin model and intermediary effect model. The results show that carbon intensity has a strong spatial and temporal correlation, showing positive spatial spillover effects and temporal path dependence; there is an obvious inverted U-shaped relationship between industrial co-agglomeration between secondary and tertiary industries and carbon intensity, which depends on the dynamic game of agglomeration effect and congestion effect. Industrial structure upgrading plays a mediating role in this process. At present, the level of industrial co-agglomeration in China has not yet crossed the inflection point, so we should optimize the labor force structure and industrial layout, and reasonably promote the development of industrial co-agglomeration between secondary and tertiary industries.