Ag@ZnS nanoparticles display enhanced photocatalytic efficiency and good photoelectric properties compared to their single-component counterparts in the process of forming a core-shell structure using an Ag cluster as the inner core of a ZnS outer shell. In this study, first-principles calculations were used to investigate the structural, electronic, and optical properties of Agn@(ZnS)42 (n=6-16) core-shell nanocomposites. The calculated results show significant even-odd oscillations in the structural stability, that is, Ag@ZnS nanostructures with an even number of Agn core atoms are relatively more stable than those with an odd number of core atoms. The second-order differences in the total energies (Δ2E) and the core-shell interaction energy Ecs indicate that a Ag12@(ZnS)42 nanostructure is the most stable configuration. A significant red shift was found in Agn@(ZnS)42 nanoparticles in the absorption spectrum compared with a (ZnS)48 nanostructure, which is likely attributed to the strong electron interactions between the Ag core and the ZnS shell.