Carcinoembryonic　antigen　(CEA),　a　vital　biomarker,　plays　a　significant　role　in　the　early　diagnosis　and　prognostic　estimation　of　malignant　tumors.　In　this　study,　a　split-type　photoelectrochemical　immunoassay　for　the　sensitive　quantification　of　CEA　has　been　successfully　developed　based　on　the　target-induced　in　situ　formation　of　a　Z-type　heterojunction.　First,　gold　nanoparticle-decorated　ZnIn2S4　(AuNPs/ZnIn2S4)　composites　were　synthesized　and　used　for　the　fabrication　of　photoelectrodes.　Then,　the　detection　antibody　labeled　with　Ag　nanoparticles　was　formed　and　applied　for　the　biorecognition　of　CEA　and　subsequent　liberation　of　Ag+　ions　to　induce　the　in　situ　formation　of　Ag2S/AuNPs/ZnIn2S4,　a　Z-type　heterojunction,　on　the　photoelectrode.　The　Z-type　Ag2S/AuNPs/ZnIn2S4　heterojunction　with　effectively　promoted　separation　of　photogenerated　charge　carriers　could　lead　to　a　markedly　enhanced　photocurrent　response　and　highly　sensitive　quantification　of　CEA.　Moreover,　the　three-dimensional　spatial　structure　of　ZnIn2S4　provides　abundant　active　sites　for　the　reaction　and　exhibits　non-enzymatic　properties,　which　are　conducive　to　the　further　improvement　of　the　analytical　performance　of　CEA.　The　developed　split-type　photoelectrochemical　immunoassay　with　good　sensitivity,　satisfactory　selectivity,　reliable　stability,　wide　dynamic　linear　range　(0.01-20　ng　mL−1),　and　low　detection　limit　(7.3　pg　mL−1)　offers　valuable　insights　into　the　development　of　novel　PEC　biosensing　models　for　the　detection　of　tumor　biomarkers　and　holds　potential　application　value　in　the　field　of　disease　diagnosis.　©　2024　The　Royal　Society　of　Chemistry.