Poly(triazine　imide)　(PTI)　holds　significant　promise　for　photocatalytic　CO2　reduction　by　addressing　the　limitations　of　conventional　carbon　nitrides.　However,　its　practical　application　remains　constrained　by　a　narrow　visible-light　absorption.　Herein,　we　report　a　barbituric　acid　(BA)-mediated　copolymerization　strategy　to　engineer　pi-electron　delocalization　within　the　triazine　framework　for　broadening　light-harvesting　spectrum　and　optimizing　charge　carrier　transport.　Under　visible　light　irradiation　(lambda　＞=　400　nm),　the　optimized　PTI-BA(1.0)　photocatalyst　achieves　a　CO　evolution　rate　of　10　mu　mol　h(-1)　(333　mu　mol　g(-1)　h(-1))　with　95%　selectivity,　representing　a　5-fold　enhancement　over　pristine　PTI.　Remarkably,　the　apparent　quantum　efficiency　reaches　13.6%　at　365　nm,　underscoring　its　superior　CO2　photoconversion　capability.　Mechanistic　investigations　via　in　situ　diffuse　reflectance　infrared　Fourier　transform　spectroscopy　and　density　functional　theory　calculations　elucidate　the　energetically　favorable　pathways　for　CO2　activation,　reduction　and　CO　desorption.　This　work　not　only　provides　a　rational　design　strategy　for　modulating　the　optoelectronic　properties　of　crystalline　carbon　nitride　but　also　advances　the　development　of　high-performance　photocatalysts　for　sustainable　CO2　conversion.