Abstract: UHPLC-MS/MS was applied to the determination of amyl xanthic acid in water. The water sample (pH 9-10) was filtered with disposable needle filter of PTFE material. ACQUITY UPLC BEH C₁₈ chromatographic column was used as stationary phase, and the mixture of water(pH was adjusted to 10 with ammonia)and methanol mixed in different ratios was used as mobile phase in gradient elution. ESI^- and multi-reactions monitor mode were adopted in MS/MS. Linearity range of amyl xanthic acid was found between 0.500 μg·L^-1 and 15.0 μg·L^-1 with detection limit of 0.114 μg·L^-1. Precision of the method was tested at the concentration levels of 1.00, 8.00, 14.0μg·L^-1 amyl xanthic acid standard solutions for 6 determinations, values of RSD's found were in the range of 2.7%-3.9%. On the base of blank sample, test for recovery was made by standard addition method and values of recovery found were in the range of 95.9%-102%.