This study examines the relationship between high-speed railways (HSRs) and environmental pollution by focusing on the mediating role of environmental regulations and the moderating role of officials’ political promotion incentives. Based on a sample of 113 prefecture-level cities, with balanced panel data in China from 2009 to 2017, using the difference-in-differences (DID) model, the results show that HSRs can reduce environmental pollution via the mediating effect of environmental regulations. Additionally, high officials’ political promotion incentives can strengthen this mediating effect. A propensity score matching with difference-in-differences (PSM-DID) model is used to solve endogenous problems, and a placebo test and a parallel trend test indicate that these results are robust. This study encourages the government to rationally promote the construction of high-speed railways and expand the social advantages of high-speed railways to improve environmental regulations and reduce environmental pollution.