A hexafluorophosphate ionic liquid is used as a functional monomer to prepare a metal-organic framework (Zn-MOF). Zn-MOF is used as a template for MoS nanosheets synthesis and further carbonized to yield light-responsive ZnS/C/MoS nanocomposites. Zn-MOF, carbonized-Zn-MOF, and ZnS/C/MoS nanocomposites are characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray diffraction pattern, scanning electron microscopy (SEM), element mapping, Raman spectroscopy, X-ray photoelectron spectroscopy, fluorescence, and nitrogen-adsorption analysis. Carcinoembryonic antigen (CEA) is selected as a model to construct an immunosensing platform to evaluate the photo-electrochemical (PEC) performances of ZnS/C/MoS nanocomposites. A sandwich-type PEC immunosensor is fabricated by immobilizing CEA antibody (Ab ) onto the ZnS/C/MoS /GCE surface, subsequently binding CEA and the alkaline phosphatase-gold nanoparticle labeled CEA antibody (ALP-Au-Ab ). The catalytic conversion of vitamin C magnesium phosphate produces ascorbic acid (AA). Upon being illuminated, AA can react with photogenerated holes from ZnS/C/MoS nanocomposites to generate a photocurrent for quantitative assay. Under optimized experimental conditions, the PEC immunosensor exhibits excellent analytical characteristics with a linear range from 2.0 pg mL to 10.0 ng mL and a detection limit of 1.30 pg mL (S/N = 3). The outstanding practicability of this PEC immunosensor is demonstrated by accurate assaying of CEA in clinical serum samples.