The　electrocatalytic　CO2　reduction　reaction　(CO2RR)　is　one　of　the　most　important　electrocatalytic　reactions.　Starting　from　a　well-defined　*CO　intermediate,　the　CO2RR　can　bifurcate　into　two　pathways,　either　forming　a　hydrogenation　product　by　*　C　O　bond　hydrogenation　or　leading　to　CO　desorption　by　*　C　bond　cleavage.　However,　it　is　perplexing　why　many　dual-atom　catalysts　(DACs)　exhibit　high　CO　selectivity　in　experiments,　despite　previous　theoretical　studies　arguing　that　the　*　C　O　bond　hydrogenation　is　thermodynamically　more　favorable　than　the　*　C　bond　breaking.　Furthermore,　the　selectivity　is　contingent　upon　the　potential　and　is　perturbed　by　the　hydrogen　evolution　reaction　(HER),　which　is　far　from　clear.　Using　ab　initio　molecular　dynamics　and　a　＂slow-growth＂　sampling　method　to　evaluate　the　potential-dependent　kinetics,　we　uncover　the　selectivity　origin　of　CO2RR　to　CO　on　a　typical　NC-based　DAC　(CuFe-N6-C).　Importantly,　the　results　show　that　at　higher　CO*　coverage,　CO*　desorption　kinetics　are　accelerated,　while　the　competing　*　C　O　bond　hydrogenation　reaction　is　inhibited　at　varying　potentials.　Furthermore,　the　selectivity　of　the　HER　is　observed　to　increase　as　the　potential　decreases.　However,　at　higher　CO*　coverage,　the　energy　barrier　for　the　*　C　bond　cleavage　is　lower　than　that　for　HER,　suggesting　that　HER　is　suppressed　on　CuFe-N6-C.　Our　work　unlocks　a　long-standing　puzzle　about　the　selectivity　of　important　DAC　catalysts　for　CO2RR　and　provides　insights　for　more　effective　catalyst　design.