A sandwich "signal-off" type photoelectrochemical (PEC) immunosensor was fabricated based on a composite heterojunction of tungsten oxide/titanium oxide microspheres (WO/TiO) acting as signal amplification platform and carbon microspheres loaded by gold nanoparticles (Cs@Au NPs) utilized as the label for detecting antibody. WO/TiO had excellent photoelectric performance, and the results of Mott-Schottky plots, open-circuit voltage, and electron spin resonance spectroscopy indicated that it belonged to the Z-scheme heterojunction transfer mechanism of photogenerated carriers. To achieve the sensitization of PEC immunosensor, Cs@Au NP-labeled immunocomplex can effectively reduce the photocurrent signal. The PEC immunosensors were fabricated under the optimal conditions of 1:1 WO/TiO (molar ratio), 2.0 mg mL WO/TiO, and 1.5 mg mL Cs@Au NPs. Through comparison of the detection results of label-free and sandwich-type PEC immunosensors for nucleocapsid (N) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), we found that the sensitivity of the sandwich type was 2.53 times the label-free type, and the limit of detection was 0.006 ng mL, i.e., 3.17 times lower than the label-free type. This demonstrates that the developed sandwich-type PEC immunosensor will have a brighter application prospect.