Light-driven　fixation　of　CO2　in　organics　has　emerged　as　an　appealing　alternative　for　the　synthesis　of　value-added　fine　chemicals.　Challenges　remain　in　the　transformation　of　CO2　as　well　as　product　selectivity　due　to　its　thermodynamic　stability　and　kinetic　inertness.　Here　we　develop　a　boron　carbonitride　(BCN)　with　the　abundant　terminal　B/N　defects　around　the　mesoporous　walls,　which　essentially　enhances　surface　active　sites　as　well　as　charge　transfer　kinetics,　boosting　the　overall　rate　of　CO2　adsorption　and　activation.　In　this　protocol,　anti-Markovnikov　hydrocarboxylation　of　alkenes　with　CO2　to　an　extended　carbon　chain　is　achieved　with　good　functional　group　tolerance　and　specific　regioselectivity　under　visible-light　irradiation.　The　mechanistic　studies　demonstrate　the　formation　of　CO2　radical　anion　intermediate　on　defective　boron　carbonitride,　leading　to　the　anti-Markovnikov　carboxylation.　Gram-scale　reaction,　late-stage　carboxylation　of　natural　products　and　synthesis　of　anti-diabetic　GPR40　agonists　reveal　the　utility　of　this　method.　This　study　sheds　new　insight　on　the　design　and　application　of　metal-free　semiconductors　for　the　conversion　of　CO2　in　an　atom-economic　and　sustainable　manner.