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.