The　photocatalytic　production　of　syngas　using　a　noble-metal-free　catalytic　system　is　a　promising　approach　for　renewable　energy　and　environmental　sustainability.　In　this　study,　we　demonstrate　an　efficient　catalytic　system　formed　by　integrating　Co　single　sites,　which　act　as　the　active　sites,　in　covalent　triazine　frameworks　(CTFs),　which　act　as　the　photoabsorber,　for　the　photocatalytic　production　of　syngas　from　CO2　in　aqueous　solution.　The　enhanced　light　absorption　of　the　CTFs,　which　contain　intramolecular　heterojunctions,　in　conjunction　with　0.8　mmol　L-1　of　the　Co　complex　enables　excellent　syngas　production　with　a　yield　of　3303　mu　mol　g(-1)　(CO:H-2　=　1.4:1)　in　10　h,　which　is　about　three　times　greater　than　that　achieved　using　CTF　without　a　heterojunction.　In　the　photocatalytic　reaction,　the　coordinated　single　Co　centers　accept　the　photogenerated　electrons　from　the　CTF,　and　serve　as　active　sites　for　CO2　conversion　through　an　adsorption-activation-reaction　mechanism.　Theoretical　calculations　further　reveal　that　the　intramolecular　heterojunctions　highly　promote　photogenerated　charge　separation,　thus　boosting　photocatalytic　syngas　production.　This　work　reveals　the　promising　potential　of　CTFs　for　single-metal-site-based　photocatalysis.　(C)　2021,　Dalian　Institute　of　Chemical　Physics,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.　All　rights　reserved.