The invention discloses a photocatalyst and a preparation method and application thereof, and belongs to the technical field of semiconductor photocatalytic decomposition of water to produce hydrogen, and the preparation method comprises the following steps: S1, preparing ZnO nanoparticles, S2, preparing ZnIn2S4 nanosheets, S3, preparing an AZnIn2S4/BZnO heterojunction nanomaterial, adopting a one-step hydrothermal synthesis method, weighing zinc chloride, indium trichloride tetrahydrate and thioacetyl, dissolving the zinc chloride, the indium trichloride tetrahydrate and the thioacetyl into ethylene glycol, adding a catalyst, and stirring to obtain the photocatalyst. The preparation method comprises the following steps: dispersing ZnO nanoparticles prepared in the step S1 into a mixed solution, carrying out ultrasonic treatment at room temperature for 25-40 minutes to form a mixed solution, dispersing the ZnO nanoparticles prepared in the step S1 into the mixed solution obtained in the step S1, carrying out ultrasonic treatment at room temperature for 25-40 minutes, then transferring into a high-pressure kettle with a polytetrafluoroethylene lining, heating at 100-130 DEG C for 1.5-3 hours to obtain a precipitate, and then centrifuging, washing and separating the precipitate obtained in the step S1 to obtain the ZnO nano-particles. The preparation method comprises the following steps: adding BZnO and AZnIn2S4 into a hydrothermal reaction kettle, carrying out vacuum drying for 10-14 hours to obtain an AZnIn2S4/BZnO heterojunction nano material which is used as a photocatalyst, and synthesizing a ZnIn2S4/ZnO heterojunction through a hydrothermal method to form the photocatalyst with high photocatalytic performance.