Water soluble L cysteine modified ZnS∶Mn2+ quantum dots（QDs） were prepared in aqueous solution under the room temperature,and characterized by infrared spectrometry,ultraviolet-visible spectrometry,fluorescence spectrometry and X-ray diffraction spectrometry.Based on the fluorescence quenching of ZnS∶Mn2+ QDs caused by folic acid（FA） ,the prepared QDs were evaluated as a fluorescence probe for FA detection.Factors affecting the fluorescence detection on FA were studied.Under the optimal conditions,the quenched fluorescence intensity was linear with FA concentration in the range of 1.0×10-6-7.0×10-5 mol?L-1 with correlation coefficient of 0.994.The limit of detection for FA was 9.6×10-7 mol?L-1.The proposed method was applied in the determination of folic acid in folic acid tablet and urine samples with satisfactory results.The mechanism for detection of FA was also investigated.