Modification　of　g-C3N4　with　metal-free　biomaterials　through　an　environmentally　friendly,　low-energy,　facile,　and　rapid　single-step　method　is　desired　for　the　preparation　of　photocatalysts　with　efficient　activity　and　high　selectivity　of　CO2　reduction　but　remains　a　great　challenge.　Herein,　we　develop　a　phase-transitioned　protein　modification　strategy　for　photocatalysts　through　superfast　amyloid-like　protein　assembly　on　surfaces　using　a　one-step　sequential　coating　method.　Metal-free　carbon　nitride/protein　heterojunction　composite　photocatalysts　(the　phase-transitioned　lysozyme　(PTL),　phase-transitioned　bovine　serum　albumin　(PTB),　and　phase-transitioned　ovalbumin　(PTO)-coated　carbon　nitride@SiO2　(CN@SiO2)　and　bioinspired　carbon　nitride　hollow　nanospheres　(CN-HS)　obtained　by　etching　of　CN@SiO2)　are　prepared　using　lysozyme,　bovine　serum　albumin,　and　ovalbumin.　The　insulator-semiconductor　heterojunctions　formed　at　the　protein-carbon　nitride　interface　promote　the　migration　and　separation　of　photogenerated　charges.　The　exposed　hydrophobic　alkyl　and　aryl　groups　of　the　surface-modified　protein　enable　the　formation　of　a　CO2-aqueous　solution-photocatalyst　three-phase　interface　on　the　catalyst　surface　and　the　exposed　−NH2　groups　provide　sites　for　CO2　adsorption,　which　effectively　increases　CO2　mass　transfer　and　its　adsorption　as　well　as　hydrophobicity,　promoting　CO2　reduction　and　inhibiting　hydrogen　production.　Therefore,　protein　modification　effectively　improves　the　CO2　reduction　activity　and　CO　selectivity.　For　instance,　compared　to　CN-HS,　the　CO　yield　of　the　PTL-modified　CN-HS　(1346.5　μmol　g-1)　increased　by　24.5　times　and　the　CO　selectivity　reached　90.5%.　These　findings　represent　a　critical　advancement　in　the　surface　modification　of　carbon　nitride　for　CO2　reduction　and　the　design　of　bioinspired　materials　for　various　applications.　©　2024　American　Chemical　Society.