Defect　engineering　as　an　important　surface　regulation　method,　is　closely　related　to　the　adsorption,　activation　and　transformation　of　reactant　molecules.　To　improve　the　efficiency　of　photocatalytic　CO2　reduction　from　the　interfacial　perspective,　g-C3N4/Zn0.3Cd0.7S　(CN/ZCS)　heterojunction　with　dual　vacancies　were　successfully　synthesized.　The　production　of　CO　for　the　optimal　sample　(3%　CN/ZCS)　is　significantly　improved,　about　3.5　times　of　ZCS　and　309.8　times　of　CN,　respectively.　Meanwhile,　the　CO2　adsorption　and　transformation　process　are　deduced　and　revealed　by　monitoring　the　intermediate　and　final　products　on　the　in-situ　FTIR.　The　underlying　enhanced　reason　can　be　elucidated　by　the　defective　sites　and　Z-scheme　transfer　model,　which　increase　the　carrier　density,　active　sites　and　the　utilization　of　photogenerated　charge　carriers　(PCCs).　This　study　provides　a　new　and　powerful　example　in　designing　novel　heterojunction　photocatalysts　by　defect　engineering,　and　helps　us　to　enhance　the　photocatalytic　activity　from　the　interfacial　perspective.