Abstract: The water-soluble CdSe/ZnS nanocrystals were synthesized using a small organic molecule with benzene ring (thiosalicylic acid) as modifier. The optical properties of CdSe and CdSe/ZnS were studied by fluorescence and UV-Vis absorption spectroscopy. The more remarkable enhancement of emision intensity for CdSe/ZnS nanocrystals was observed than for CdSe nanocrystals. The optimal conditions for synthesis are with a molar ratio of CdSe and ZnS of 1:8, pH 11.0 and refluxing for 45 min at 80 0C. The fluorescence intensity has been stable for 3 months. The shape of CdSe/ZnS nanocrystals showed unique dendritic structure under TEM. Under the optimal conditions, the fluorescent intensity in the system of CdSe/ZnS NCs-BSA is linearly proportional to the concentration of BSA from 2.56 to 15.00 mg L-1. The linear equation is F=101.2C-11.3(C, mg L-1), R＝0.9962, with a detection limit of 0.64 mg L-1.