Polymeric　carbon　nitrides　(PCNs)　are　attractive　candidates　for　diverse　photoredox　catalysis,　but　the　CO2　reduction　activity　of　pristine　PCNs　is　unsatisfactory,　mainly　due　to　the　bulky　structure　and　insufficient　optical　harvesting.　In　this　work,　a　facile　and　efficient　strategy　coupling　copolymerization　and　stream　reforming　is　developed　to　exfoliate　the　bulky　PCNs　into　PCN　nanosheets　(named　PCN-T-NSs).　The　ameliorated　morphological　and　electronic　structures　of　PCN-T-NSs　enable　the　exposure　of　abundant　catalytically-active　sites,　accelerated　separation　of　photogenerated　charge　carriers,　and　strengthened　photoabsorption　in　the　visible　region.　Consequently,　the　optimized　PCN-T-NS　photocatalyst　shows　a　greatly　enhanced　CO2　reduction　activity,　with　a　remarkable　CO-releasing　rate　of　43　mu　mol　h(-1),　which　is　about　40　times　greater　than　that　of　pristine　PCN.　Besides,　a　high　apparent　quantum　efficiency　(AQE)　of　4.2%　is　realized　by　the　PCN-T-NS　catalyst　under　420　nm　photoirradiation　with　TEOA　as　a　sacrificial　reagent　and　Co(bpy)(3)(2+)　as　a　cocatalyst.　Moreover,　the　integrated　copolymerization　and　stream　reforming　strategy　is　extendable　to　the　assembly　of　several　other　carbon　nitride　nanosheets　with　enhanced　performance　for　CO2　photoreduction　by　employing　the　typical　comonomers.　This　work　may　bring　some　fresh　vitality　for　constructing　well-tailored　carbon　nitride　polymers　with　functions　for　sustainable　solar-to-chemical　energy　conversion.