The water-soluble Ag+-doped ZnS nanocrystals surface capped with cysteine (expressed as ZnS:Ag/Cys) were synthesized in aqueous solution by using l-cysteine as surface modifier. The crystal structure, size, shape, component, and spectral properties of ZnS:Ag/Cys nanocrystals were characterized by X-ray power diffraction, transmission electron microscope, energy dispersive X-ray analysis, inductively coupled plasma atomic emission spectrometry, infrared spectrum, UV-Vis absorption spectrum, and photoluminescence spectrum. The results show that the spherical ZnS:Ag/Cys nanocrystals with an average diameter of 2.6 nm have good fluorescent characteristics, their fluorescence intensity is enhanced greatly after doped with Ag+. And the sulfur atoms in cysteine molecules are coordinated with metal ions on the surface of the nanocrystals, the cysteine modified on the surface of ZnS:Ag/Cys nanocrystals renders the nanocrystals water soluble and biocompatible. The ZnS:Ag/Cys nanocrystals have potential applications in molecular assembly and biological fluorescence analysis.