Full　mobilization　and　effective　use　of　climate　aid　is　of　great　importance　for　the　low　carbon　transition　of　Belt　and　Road　Initiative　(BRI)　countries.　This　study　utilizes　the　two-step　system　generalized　method　of　moments　model　to　evaluate　the　effects　of　climate　aid　on　carbon　emissions　reduction　and　the　impact　mechanism　through　energy　structure　optimization.　The　panel　data　of　93　Belt　and　Road　countries　from　2000　to　2018　were　used　for　empirical　analysis.　Results　show　that　climate　aid　has　a　significant　reduction　effect　on　the　carbon　emissions　intensity　of　BRI　countries,　and　the　dominant　component　of　climate　aid,　i.e.,　mitigation　aid,　corresponds　to　better　carbon　emissions　reduction　benefits　than　adaptation　aid.　The　impact　mechanism　demonstrates　that　the　climate　aid　has　dual　carbon　emissions　reduction　effects　in　BRI　countries　which　have　an　intermediate　energy　structure.　It　indicates　that　climate　aid　not　only　directly　reduces　carbon　emissions　by　increasing　carbon　reduction　resources,　but　also　indirectly　reduces　carbon　emissions　by　promoting　renewable　energy　and　optimizing　the　energy　structure.　The　results　evidence　the　theory　of　environmental　Kuznets　curve　hypothesis　and　channels　of　climate　aid　effects.　Practical　implications　from　the　current　study　include　that　more　climate　finance　support　should　be　provided　to　BRI　countries　by　developed　countries,　the　effectiveness　of　climate　aid　should　be　improved　by　investing　in　projects　with　significant　carbon　reduction　like　renewable　energy,　data　monitoring　and　performance　evaluation　of　climate　aid　should　be　strengthened,　and　China　should　take　its　role　and　make　more　contributions　to　the　low　carbon　transition　especially　energy　transition　of　BRI　countries.