Heat　transfer　characteristics　analysis　of　supercritical　carbon　dioxide　(sCO2)　in　variable　cross-section　tubes　is　critical　for　its　promotion　and　application.　This　paper　proposed　three　horizontal　heat　exchange　tubes,　i.e.,　straight　tube,　converging　tube,　and　diverging　tube.　On　this　basis,　we　used　numerical　simulation　to　analyze　the　convective　heat　transfer　performance　of　sCO2　in　the　horizontal　tube　with　variable　cross-section　under　heating　conditions.　The　diverging　tube　effectively　enhances　the　overall　heat　transfer　performance　compared　with　the　uniform　cross-section　tube,　and　the　heat　transfer　rate　is　increased　by　19.26%　compared　with　that　of　the　straight　tube.　In　contrast,　the　converging　tube　weakens　the　heat　transfer　ability.　Moreover,　we　proposed　quasi-air　film　and　eddy　blockage　concepts　to　reveal　the　fluid＇s　heat　transfer　deterioration　mechanism　near　the　top　bus.　Wherein　the　quasi-air　film　is　full　of　sCO2,　which　has　a　low　thermal　conductivity　close　to　that　of　air　at　room　temperature,　and　generates　near　the　top　bus.　Besides,　the　criterion　factor,　But　was　proposed　by　double　corrections　of　density　and　temperature　for　judging　the　occurrence　of　eddy　blockage,　and　the　Nu-based　evaluation　shows　that　the　eddy　blockage　can　deteriorate　the　heat　transfer　performance　at　the　top　of　the　horizontal　tube　by　17%.　The　physical　mechanism　of　heat　transfer　deterioration　under　the　heating　condition　is　clarified　by　the　dual-effect　of　quasi-air　film　and　eddy　blockage　on　this　account.　©　2021　Elsevier　Ltd