With　China’s　continuous　urbanization　and　industrialization,　the　emissions　in　Chinese　cities　have　become　increasingly　serious.　Chinese　government　has　set　a　goal　to　reach　carbon　peak　and　achieve　carbon　neutrality,　endeavoring　to　gradually　realize　net-zero　carbon　dioxide　(CO2)　emission.　Moreover,　the　new　transport　technology　has　also　been　fast　developed,　represented　by　the　quick　expansion　of　the　high-speed　rail　(HSR)　system.　Based　on　the　Air　Quality　Index　(AQI)　of　286　Chinese　cities　over　the　2016–2019　period,　this　paper　first　adopts　the　spatial　auto-correlation　analysis　to　quantify　the　spatio-temporal　characteristics　of　Chinese　cities’　AQI.　Then,　a　spatial　difference-in-differences　(SDID)　model　is　estimated　to　shed　light　on　how　Chinese　cities’　air　quality　can　be　affected　by　HSR.　Our　paper　identifies　apparent　spatio-temporal　distribution　patterns　in　Chinese　cities’　air　quality.　Our　empirical　results　that　the　HSR　opening　can　help　reduce　emissions　to　improve　the　city’s　air　quality.　Moreover,　HSR　opening　in　the　adjacent　city　can　also　improve　one　city’s　air　quality　(i.e.,　the　neighboring　effect).　We　also　highlight　and　verify　the　mechanism　of　such　a　positive　HSR　impact　on　the　city’s　air　quality.　First,　as　a　cleaner　transport　mode,　HSR　helps　divert　traffic　from　other　more　polluting　modes　(i.e.,　positive　direct　“transport　substitution　effect”).　HSR　also　helps　promote　the　city’s　tertiary　industry,　leading　to　fewer　emissions　(i.e.,　positive　indirect　“industrial　structure　effect”).　Our　heterogeneous　analyses　further　demonstrate　that　HSR　is　more　effective　to　improve　the　air　quality　in　eastern　and　western　regions.　But　the　neighboring　effect　is　only　obvious　in　eastern　China　as　the　cities　are　closer　to　each　other　in　terms　of　economic　relations　and　geographic　locations.　©　2023,　The　Author(s),　under　exclusive　license　to　Springer　Nature　Switzerland　AG.