Photosynthetic　conversion　of　CO2　into　fuel　and　chemicals　is　a　promising　but　challenging　technology.　The　bottleneck　of　this　reaction　lies　in　the　activation　of　CO2,　owing　to　the　chemical　inertness　of　linear　CO2.　Herein,　we　present　a　defect-engineering　methodology　to　construct　CO2　activation　sites　by　implanting　carbon　vacancies　(CVs)　in　the　melon　polymer　(MP)　matrix.　Positron　annihilation　spectroscopy　confirmed　the　location　and　density　of　the　CVs　in　the　MP　skeleton.　In　situ　diffuse　reflectance　infrared　Fourier　transform　spectroscopy　and　a　DFT　study　revealed　that　the　CVs　can　function　as　active　sites　for　CO2　activation　while　stabilizing　COOH*　intermediates,　thereby　boosting　the　reaction　kinetics.　As　a　result,　the　modified　MP-TAP-CVs　displayed　a　45-fold　improvement　in　CO2-to-CO　activity　over　the　pristine　MP.　The　apparent　quantum　efficiency　of　the　MP-TAP-CVs　was　4.8　%　at　420　nm.　This　study　sheds　new　light　on　the　design　of　high-efficiency　polymer　semiconductors　for　CO2　conversion.