Using 3-Mercaptopropionic acid(MPA) as surface coated agent, Co2+-doped water-soluble quantum dots of ZnS were synthesized by co-precipitation method. The effects of Co2+ dopant and doping amount on microstructure, morphology and photoluminescence properties of the As-products were researched by X-ray diffraction, transmission electron microscopy, UV-visible absorption spectrometer and fluorescence spectrophotometer. The results show that the products were irregular spherical cubic zinc blender ZnS with sizes around 5.2 nm. Photoluminescence spectra analysis shows that Co2+-doped ZnS quantum dots formed a recombination centers with impurity level and defect, the luminescent properties of the doped products are increased obviously by emitting red fluorescence. The synthesis mechanism of Co2+-doped ZnS quantum dots was studied by infrared spectroscopy.