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　in　China.　However,　there　has　been　very　little　investigation　on　how　the　HSR　can　affect　city-level　emission　and　urban　air　quality.　Especially,　the　potential　spatial　spillover　effect　and　the　mechanism　of　such　potential　impact　have　not　been　examined.　Based　on　the　urban　air　quality　index　(AQI)　of　286　Chinese　cities　over　the　2016-2019　period,　this　study　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　and　the　spatial　Dubin　model　(SDM)　are　estimated　to　shed　light　on　how　Chinese　cities＇　air　quality　can　be　affected　by　HSR.　This　study　identifies　apparent　spatio-temporal　distribution　patterns　in　Chinese　cities＇　air　quality.　We　found　that　the　opening　of　HSR　in　Chinese　cities　significantly　improves　the　urban　air　quality　by　reducing　AQI　by　an　average　of　4%.　Moreover,　the　spatial　auto-correlation　coefficient　is　0.0897　and　statistically　significant.　HSR　opening　in　the　adjacent　city　can　also　improve　one　city＇s　AQI　by　an　average　of　1.3%　(i.e.,　the　neighboring　effect).　We　also　highlighted　and　verified　the　mechanism　of　such　a　positive　HSR　impact　on　the　urban　air　quality.　First,　as　a　cleaner　transport　mode,　HSR　helps　divert　traffic　from　other　more　polluting　modes.　HSR　also　helps　promote　the　city＇s　tertiary　industry,　leading　to　fewer　emissions.　The　hetero-geneous　analyses　further　demonstrated　that　HSR　is　more　effective　to　improve　the　urban　air　quality　in　eastern　region　(with　the　AQI　reduction　of　5.88%)　and　western　regions　(with　AQI　reduction　of　7.25%).