The present invention provides an Ag/ZnS nano composite SERS substrate and a preparation method thereof, and relates to the technical field of nano composite materials. The substrate introduces a semiconductor ZnS with a relatively wide band gap to contact with Ag, uses a modified 4-mercaptobenzoic acid (4-MBA) as a probe, explores photoelectric properties of a model through regulating a band gapstructure, a surface defect state and a surface electronic state of the semiconductor ZnS, and adjusting excitation wavelength to obtain an SERS effect. Metals exhibits a characteristic that free electrons occupying a highest Fermi level transfer charges to adsorbed molecules and a large number of bounding electrons, all changing optical properties of the free electrons. Interactions between a semiconductor energy band and molecular energy level leads to new charge transfer behavior. The optoelectronic properties provide good evidence for research and better understanding that charge transferprocess at an interface in an SERS enhancement mechanism.