Abstract: The carbon dots (CDs) with discoloration property was synthesized by hydrothermal method using citric acid as carbon source and urea as nitrogen source. Based on the discoloring reaction between CDs and S^2-, a method for the determination of S^2- in desulfurization solution and waste water by UV-Vis spectrophotometry was proposed. The cyclic pretreatment system was used to separate S^2- from the sample. An aliquot (0.5 mL) of the sample solution for test and 4.5 mL of 100 mg·L^-1CDs solution were mixed well, and the acidity was adjusted to pH 8 for reaction at 30 ℃ for 15 min, then the mixed solution was rapidly cooled to room temperature. The absorbance of the system at 610 nm was determined by UV-Vis spectrophotometry before and after the reaction. It was shown that the discoloration of CDs was caused by oxidation, and the selective discoloring reaction of CDs was induced by S^2- at pH 8. Linear relationships between values of mass concentration of S^2- and corresponding difference of absorbance were kept in the ranges of 0.05-1.00 mg·L^-1 and 1.00-15.00 mg·L^-1, with detection limits (3S/N) of 0.043 mg·L^-1and 0.073 mg·L^-1. Test for recovery was made on the actual samples, giving results of in the range of 98.0%-106%, with RSDs (n=6) of the determined values ranged from 0.92% to 1.1%. This method was applied to the analysis of the actual desulfurization solution and waste water samples, and there was no significant difference with GB/T 16489-1996.