The invention provides a preparation method of an ultrathin molybdenum disulfide nanosheet/silicon nanowire (SiNWs/MoS2) heterojunction structure. The preparation method comprises the following stepsthat S1, silicon wafers are cleaned; S2, the etching of a silicon nanowire array is assisted through the catalysis of metal nano particles; and S3, by taking the silicon nanowire array obtained in theS2 as a substrate, and molybdenum oxide and sulfur powder as a precursor, annealing growth is carried out, and the ultrathin molybdenum disulfide nanosheet/silicon nanowire heterojunction structure is obtained. Through the construction of the ultrathin molybdenum disulfide (MoS2) nanosheet/silicon nanowire heterojunction structure, the catalytic active site density of molybdenum disulfide can beimproved effectively, the photocatalysis hydrogen production efficiency of silicon nanowires is improved effectively and the catalysis stability of the silicon nanowires can be improved. A preparationprocess of the preparation method is simple and easy to implement, the cost is low, and mass production is easy.