Based on panel data on 248 prefecture-level cities in China from 2003 to 2018, this study first estimates the treatment effect of the Shanxi Comprehensive Reform Area policy on PM 2.5 concentrations using a PSM-DID method. The empirical results show that, on average, the Shanxi Comprehensive Reform Area policy significantly increased the PM 2.5 concentrations of prefecture-level cities in Shanxi Province by 0.211% annually, and the place-based placebo test shows that the treatment effect obtained above is robust. Second, the dynamic effects show a continuous decrease in incremental effects during 2011–2018, gradually decreasing from a significant positive increment during 2011–2015 to a zero or even a negative increment during 2016–2018, indicating that the Shanxi Comprehensive Reform Area policy gradually increased in environmental friendliness. Third, the influencing mechanisms show that the Shanxi Comprehensive Reform Area policy influenced PM 2.5 concentrations by increasing the intensity of resource exploitation and decreasing the intensity of environmental regulations, but the capacity of scientific and technological innovations had no mediating effect on the relationship between the policy and PM 2.5 concentrations. Therefore, the government should further reduce the intensity of resource exploitation, strengthen the intensity of environmental regulations, and promote environmentally focused scientific and technological innovations to reduce PM 2.5 concentrations in Shanxi Province.