Carbon　emission　efficiency,　which　is　mainly　affected　by　economic　output,　energy　efficiency　and　energy　structure,　is　the　effect　of　carbon　emissions　generated　in　economic　activities.　Improving　carbon　emission　efficiency　and　narrowing　regional　differences　are　very　important　for　green　development　in　Belt　and　Road　Initiative　regions　with　developing　countries　as　the　main　body.　The　existing　literature　mostly　uses　the　Theil　index　to　study　the　temporal　differences　in　carbon　emission　efficiency,　but　spatial　differences　and　the　reasons　for　the　differences　have　rarely　been　examined.　This　paper　measures　the　differences　of　carbon　emission　efficiency　using　the　Theil　index　and　examines　the　reasons　based　on　the　Logarithmic　Mean　Divisia　index　(LMDI)　method　in　five　groups　of　60　Belt　and　Road　Initiative　countries.　The　results　show　that　the　Theil　index　of　carbon　emission　efficiency　in　these　countries　is　0.196,　with　an　intra-group　difference　of　0.165　and　an　inter-group　difference　of　0.031.　Between　most　of　the　groups,　energy　efficiency　is　the　dominant　factor　affecting　carbon　emission　efficiency　differences,　especially　between　East　Asia　and　Central　and　Eastern　Europe,　South　Asia　and　East　Asia.　Between　most　of　the　countries　with　the　highest　and　lowest　carbon　emission　efficiency　in　the　same　group,　such　as　Singapore　and　Vietnam　in　East　Asia,　energy　efficiency　is　still　the　primary　factor　affecting　the　differences.　Only　some　differences　between　a　few　groups,　countries　and　sectors　have　been　caused　mainly　by　energy　structures.　Therefore,　improving　energy　efficiency　is　the　first　way　for　those　countries　with　low　carbon　emission　efficiency　to　catch　up　other　countries　with　high　carbon　emission　efficiency,　followed　by　improving　the　energy　structure.