Infrastructure　construction　results　in　transportation　cost　and　regional　economy　changes.　Based　on　the　exogenous　infrastructure　change　of　a　county　highway　opening　as　a　natural　experiment,　this　paper　uses　the　spatial　difference-in-differences　(DID)　model　to　analyze　the　impact　of　highway　construction　in　three　different　provinces　of　China　from　2004　to　2017.　It　was　proved　that　highway　connectivity　had　no　significant　or　negative　impact　on　the　economy　of　the　counties　in　three　provinces　that　the　highway　passes　through.　In　addition,　highway　connectivity　was　claimed　to　have　obvious　spatial　spillover　effects,　which　could　promote　the　economic　development　of　the　whole　region.　For　counties　belonging　to　provinces　in　different　regions　and　stages　of　economic　development,　we　establish　a　spatial　DID　model　that　integrates　time　and　space　dimensions　to　study　the　impact　of　expressways　on　county　economic　development,　to　make　up　for　the　traditional　DID　model＇s　dependence　on　SUTVA　assumptions,　and　to　obtain　spatial　spillover　effects.