<p>The main goal of institutional openness is to expand opening up to the outside world, but it has also had a positive impact on enhancing the synergistic effect of air pollution reduction and carbon reduction. This paper innovatively uses the policy of free trade pilot zones to measure institutional openness. Furthermore, based on panel data from 284 Chinese cities from 2012 to 2021, using the difference-in-differences approach, this paper empirically tested whether institutional openness enhance the synergistic effect of air pollution reduction and carbon reduction, as well as its heterogeneity, mechanism, and spatial spillover effects. The study found that institutional openness can significantly enhance the synergistic effect of air pollution reduction and carbon reduction. After a series of robustness tests, this conclusion remains robust. Heterogeneity analysis found that these impacts are especially noticeable in eastern cities, non-resource-based cities, and high-level local government administrative efficiency cities. Mechanism analysis found that upgrading industrial structures, energy efficiency, green technology innovation, and public environmental awareness are the critical mechanisms for realizing this process. Spatial spillover effects found that institutional openness significantly reduces the synergistic effect of air pollution reduction and carbon reduction in adjacent areas.</p>