Climate　anomalies　are　affecting　the　world.　How　to　reduce　carbon　emissions　has　become　an　important　issue　for　governments　and　academics.　Although　previous　researchers　have　discussed　the　factors　of　carbon　emission　reduction　from　environmental　regulation,　economic　development,　and　industrial　structure,　limited　studies　have　explored　the　carbon　emission　reduction　effect　of　a　city’s　spatial　structure.　Based　on　108　Chinese　cities　from　the　Yangtze　River　Economic　Belt　between　2003　and　2017,　this　paper　examines　the　impact　of　the　city　cluster　policy　on　city　carbon　emissions　using　the　difference-in-differences　(DID)　method.　We　find　that:　(1)　The　city　cluster　policy　has　significantly　reduced　the　cities’　carbon　emissions　by　7.4%.　Furthermore,　after　a　series　of　robust　and　endogenous　tests,　such　as　parallel　trend　and　PSM-DID,　the　core　conclusion　still　remains.　(2)　We　further　identify　possible　economic　channels　through　this　effect,　and　find　that　city　cluster　policy　would　increase　city　productivity,　city　technological　innovation,　and　industrial　structure　optimization.　The　conclusions　of　this　paper　have　important　practical　significance　for　China　to　achieve　carbon　neutrality　and　facilitate　future　deep　decarbonization.　©　2022　by　the　authors.