Urban　spatial　compactness　is　crucial　for　reducing　carbon　emissions　and　promoting　sustainable　urban　development.　However,　the　potential　of　and　variations　in　spatial　compactness　in　county-level　carbon　reduction　have　been　largely　overlooked.　Taking　the　county-level　areas　in　the　Beijing-Tianjin-Hebei　(BTH)　region　as　an　example,　after　reviewing　the　dimensions,　scales,　and　pathways　of　the　existing　research,　we　quantifies　the　spatial　compactness　from　2005　to　2015　in　terms　of　density,　morphology,　and　function.　The　results　reveal　that,　firstly,　BTH　county-level　areas＇　carbon　emissions,　both　per　land　area　(CO2/L)　and　per　capita　(CO2/P),　have　generally　risen.　Heterogeneity　varied　across　spatial　compactness　indicators:　urban　density　and　functional　compactness　increased,　while　morphological　compactness　showed　a　minor　fluctuation.　Second,　the　correlations　with　CO2/L　were　stronger　than　those　with　CO2/P,　ranking　from　urban　density,　functional　compactness　＞　morphological　compactness.　Additionally,　the　geographically　and　temporally　weighted　regression　(GTWR)　model　revealed　the　spatial　and　temporal　heterogeneity　in　the　impact　of　spatial　compactness　on　carbon　emission　intensity.　The　heterogeneity　results　provide　insights　into　the　mechanisms　and　general　patterns　of　the　differences　in　the　factors　influencing　carbon　emissions　in　four　different　types　of　county-level　areas,　namely,　central,　eastern,　southern,　and　northwestern,　offering　policy　guidance　to　promote　the　reductions　in　both　CO2/L　and　CO2/P.　This　study　is　helpful　for　proposing　urban　spatial　planning　strategies　for　carbon　reduction　and　formulating　differentiated　carbon　reduction　policies.