The structure stability and electronic and optical properties of a series of Au@ZnS core-shell nanocomposites with different sizes were investigated theoretically by first-principle calculation based on density functional theory (DFT).A series of Aun@(ZnS)42 structures with different n values from 6 to 16 were optimized and obtained.Based on the core-shell interaction energy and second-order difference of total energy of these structures,it is found that Au13@(ZnS)42 turns out to be the most stable structure.Based on the model of Au13@(ZnS)42,the density of state and charge density difference were studied and provided a deeper understanding of the electronic structures of Au@ZnS.On the other hand,absorption coefficient and dielectric function were investigated to study the optical properties.It is found that an optical absorption peak appears in visible-light region,indicating that the photo-catalytic can be improved prominently due to the optical redshift to visible-light region when forming core-shell structure from ZnS bulk.And the redshift phenomenon accords well with experiment.Furthermore,the electronic structure further confirms the existence of redshift of optical absorption spectrum.