The　electrocatalytic　reduction　reaction　of　CO2　(CO2RR)　is　a　promising　strategy　to　promote　the　global　carbon　balance　and　combat　global　climate　change.　Herein,　exclusive　Bi-N-4　sites　on　porous　carbon　networks　can　be　achieved　through　thermal　decomposition　of　a　bismuth-based　metal-organic　framework　(Bi-MOF)　and　dicyandiamide　(DCD)　for　CO2RR.　Interestingly,　in　situ　environmental　transmission　electron　microscopy　(ETEM)　analysis　not　only　directly　shows　the　reduction　from　Bi-MOF　into　Bi　nanoparticles　(NPs)　but　also　exhibits　subsequent　atomization　of　Bi　NPs　assisted　by　the　NH3　released　from　the　decomposition　of　DCD.　Our　catalyst　exhibits　high　intrinsic　CO2　reduction　activity　for　CO　conversion,　with　a　high　Faradaic　efficiency　(FECO　up　to　97%)　and　high　turnover　frequency　of　5535　h(-1)　at　a　low　overpotential　of　0.39　V　versus　reversible　hydrogen　electrode.　Further　experiments　and　density　functional　theory　results　demonstrate　that　the　single-atom　Bi-N-4　site　is　the　dominating　active　center　simultaneously　for　CO2　activation　and　the　rapid　formation　of　key　intermediate　COOH*　with　a　low　free　energy　barrier.