The invention relates to a preparation method of a photoelectrochemical immunosensor based on bismuth molybdate/zinc doped cadmium sulfide/gold. According to the invention, a zinc-doped cadmium sulfide-sensitized bismuth molybdate nanosheet is used as a photosensitive substrate material; bismuth molybdate is simple to prepare and has a good photocatalytic effect; after sensitization of zinc ion-doped cadmium sulfide, good photoelectric response is acquired; gold nanoparticles are used to further modify the sensitized substrate photosensitive material; on one hand, the band gap defect of cadmium sulfide is improved after zinc doping, which makes the surface plasmon effect of gold more prominent; the vibration of electrons on the surface of the photosensitive material is promoted; electron transfer is accelerated to acquire more stable and excellent photocurrent; on the other hand, Au-NH2 bond is easily formed between gold and biomolecules, which provides the basis for the connection between an inorganic semiconductor nanomaterial and biomolecules, and improves the stability and sensitivity of the sensor; and the detection limit of the sensor is up to 0.03 pg/mL.