Enhancing the photoelectric (PE) response is crucial for improving the performance of solar cells, photocatalysis, and photoelectrochemistry (PEC). The coupling of different semiconductor materials to form heterostructures is a sophisticated strategy for endowing them with promising electronic properties to promote the generation and migration of carriers. Herein, the two-dimensional (2D) SnS₂/Ag₂S heterojunction with vertical state was successfully constructed, which effectively boosted the photocurrent. Based on antibody-antigen interaction and the PE mechanism, the biofunctional PEC immunoassay platform was developed for the sensitive determination of the avian influenza virus. It is found to exhibit an excellent detection range from 0.5 to 1 × 10⁴ pg mL^-1 with 0.07 pg mL^-1 detection limits. In addition, the growth and photochemical mechanism were thoroughly analyzed. This work validates a compelling approach for designing efficient PEC junctions and bioassay applications.