Abstract: A method of spectrophotometry for the determination of cysteine in medicines using Cu(Ⅱ) as probe was proposed. It was found that in a acetate buffer solution of pH 4.0, Cu(Ⅱ) was reduced to Cu(Ⅰ) by mercapto group (-SH) of cysteine, and Cu(Ⅰ) produced was precipitated with SCN- as CuSCN. Amount of cysteine was calculated indirectly from the analytical results of Cu(Ⅱ) present in the solution. Linear relationship between values of absorbance of solution and mass concentration of cysteine was kept in the range of 0.20-9.0 mg·L-1, and value of apparent molar absorptivity found was 6.05×104 L·mol-1·cm-1. The proposed method was applied to the determination of cysteine in simulative water, giving values of recovery and RSD′s (n=6) in the ranges of 96.0%-103% and 0.67%-1.4% respectively.