A　signal-on　photoelectrochemical　(PEC)　biosensor　was　successfully　established　for　the　sensitive　monitoring　of　carcinoembryonic　antigen　(CEA)　by　using　copper　indium　disulfide-sensitized　graphitic-like　carbon　nitride　(g-C3N4/CuInS2)　as　the　photosensitive　material　and　cobalt　oxyhydroxide　(CoOOH)　as　the　light-blocking　material,　coupling　target-triggered　hybridization　chain　reaction　(HCR)　with　hydrolysate-induced　dissolution/etching　of　CoOOH　nanosheets.　Initially,　a　sandwiched　reaction　occurred　between　capture　aptamer-conjugated　magnetic　bead　and　trigger　aptamer　in　the　presence　of　CEA.　Then,　the　carried　trigger　aptamer　initiated　HCR　between　two　hairpin　sequences　to　produce　long　double-helix　strand　for　capturing　alkaline　phosphatase.　The　generated　ascorbic　acid　reduced/etched　CoOOH　nanosheets　into　divalent　cobalt　ions,　which　decreased　the　amount　and　thickness　of　CoOOH　and　exposed　the　underlying　g-C3N4/CuInS2,　thus　leading　to　a　distinct　increase　in　the　photocurrent.　Under　optimum　conditions,　PEC　sensor　showed　high　sensitivity　toward　CEA　with　a　dynamic　range　of　0.02-40　ng　mL(-1)　and　a　detection　limit　of　5.2　pg　mL(-1).　Also,　it　possessed　favorable　selectivity,　high　stability　as　well　as　good　precision.　The　accuracy　of　PEC　approach　was　well　consistent　with　commercial　CEA　ELISA　kit.　These　exceptional　analytical　performances　of　PEC　biosensor　indicated　that　it　might　have　a　broad　application　prospect　in　the　diagnosis　of　CEA.