Based on the necessity and urgency of detecting infectious disease marker procalcitonin (PCT), a novel unlabeled photoelectrochemical (PEC) immunosensor was prepared for the rapid and sensitive detection of PCT. Firstly, SnO2 porous nanoflowers with good photocatalytic performance were prepared by combining hydrothermal synthesis and calcining. BiOI nanoflowers were synthesized by facile ultrasonic mixed reaction. Ag2S quantum dots were deposited on SnO2/BiOI composites by in situ growth method. The SnO2/BiOI/Ag2S composites with excellent photoelectric properties were employed as substrate material, which could provide significantly enhanced and stable signal because of the energy level matching of SnO2, BiOI and Ag2S and the good light absorption performance. Accordingly, a PEC immunosensor based on SnO2/BiOI/Ag2S was constructed by using the layered modification method to achieve high sensitivity analysis of PCT. The linear dynamic range of the detection method was 0.50 pg·mL−1~100 ng·mL−1, and the detection limit was 0.14 pg·mL−1. In addition, the designed PEC immunosensor exhibited satisfactory sensitivity, selectivity, stability and repeatability, which opened up a new avenue for the analyzation of PCT and further provided guidance for antibiotic therapy.