Abstract: The color reaction of thorium with 2, 3, 14-trihydroxyphenylazo-4-sulfonicacid is reported in this paper. In a KC1-HC1 solution of pH=2.0-2.5 in the pre-sence of TPC, a yellow complex is produced. The maximum absorbance of thesystem is found at 460 nm. The composition ratio of the complex is 1:3, and themolar absorptivity is 4.95×10~4 1.mol~(-1).cm~(-1). Beer's law is obeyed in the range of0-80 μg thorium in 25 ml solution. A great deal of U(Ⅵ), Y(Ⅲ) and La (Ⅲ) donot interfere with the determination of thorium. The method is simple and rapid.