Defect　engineering　has　being　regarded　as　one　of　the　effective　ways　to　regulate　chemical　and　electronic　structure　of　semiconductors.　Recently,　our　collaborative　work　has　shown　experimentally　that　carbon　vacancy　on　polymeric　carbon　nitride　(CV)　can　greatly　improve　the　CO2　to　CO　conversion　with　a　45-fold　improvement　over　the　polymeric　carbon　nitride　(Angew.　Chem.　Int.　Ed.,　2019,　58,　1134).　In　order　to　clarify　the　detailed　mechanism　of　promotion,　we　have　systematically　studied　the　electronic　properties　of　CV　and　hydrogenated　CV　(CV+H)　as　well　as　the　effective　CO2　reduction　reaction　through　density　functional　theory　calculations.　We　found　that　it　is　the　synergistic　effect　for　the　CO2　reduction　reaction　in　the　CV　systems,　as　the　onset　potentials　of　several　CVs　are　much　lower　than　that　of　the　polymeric　carbon　nitride.　In　particular,　the　onset　potentials　of　CV1,　CV2,　and　CV2+H　are　around　0.9　similar　to　1.5　eV　with　a　strong　chemisorbed　CO2　on　them.　Combined　with　the　analysis　of　the　electronic　properties,　our　results　confirm　that　defect　engineering　increases　the　lifetime　of　photo-generated　charges,　improves　photocatalytic　activity,　and　promotes　the　CO2　reduction　reaction　on　the　defected　polymeric　carbon　nitrides.