A sensitive photoelectrochemical (PEC) immunoassay for the carcinoembryonic antigen (CEA) is described that is based on the use of C3N4-BiOCl semiconductor on an ITO electrode. The photocurrent of the modified electrode was measured under visible light illumination. It increased in presence of L-cysteine due to rapid separation of the photoexcited electrons and holes. A sandwich-type immunoassay in a 96-well microtiter plate format used CuO nanoparticles as label for the secondary antibody. The Cu2+ is released from the CuO in the sandwich complex by treatment with acid. The free Cu2+ combined with both the cysteine and the electron receptors of C3N4 and BiOCl. Under optimal conditions, this dual action immensely decreases the photocurrent of the PEC system, and the response is inversely proportional to the CEA concentrations from 0.1 pg mL−1 to 10 ng mL−1 at the working voltage of 0 V (vs. SCE). The detection limit is 0.1 pg mL−1, and the method is exhibited satisfactory selective, repeatable and stable. Graphical abstractSchematic representation of an immunoassay based on cysteine-assisted C3N4-BiOCl photoelectrochemical platform. CuO nanoparticles were utilized as labels in immunocomplex to release Cu2+ in acidic condition. Carcinoembryonic antigen in sample was detected sensitively by dual function of Cu2+ with cysteine and C3N4-BiOCl semiconductor.