With　the　advantage　of　excellent　light　absorption　ability　and　earth-abundant　elements,　Cu2ZnSnS4　(CZTS)　is　widely　applied　in　the　photoelectrochemical　(PEC)　reduction　of　CO2.　However,　the　microscopic　mechanism　of　the　CO2　conversion　on　CZTS　is　still　unclear.　Here,　we　have　theoretically　investigated　the　electronic　properties　of　the　CZTS　to　reveal　the　special　bonding　properties　of　CO2　under　both　exposed　metal　and　sulfur　terminations　of　the　CZTS　and　the　formation　mechanisms　of　producing　CO,　HCOOH,　and　CH4　from　a　microscopic　perspective.　Our　study　shows　that　when　the　metal　is　exposed　to　the　surface,　CO2　will　convert　to　HCOOH　with　a　high　selectivity　with　the　limiting　potential　of　0.23　V　(vs　RHE).　However,　on　the　sulfur　termination,　the　onset　potentials　of　producing　HCOOH,　CO,　and　CH4　are　all　lower　than　0.4　V,　indicating　the　mix　formations.　These　findings　provide　theoretical　insight　into　the　PEC　experiments　of　CO2　reduction　on　CZTS.　©　2021　American　Chemical　Society.