Abstract: The condition for the color reaction of chloroposphonazo-pC with thorium has been studied. The experimental results show that thorium reacts with chloroposphonazo-pC to form a stable blue complex in the acid medium. The maximum absorbance of the complex is at 680 nm,the apparent molar absorptivity is ε=1.04×10⁵ L/(mol·cm). Beer's law is obeyed in the range of 0.0～10.0/10 mL,the ratio thorium to chloroposphonazo-pC in the complex is 1:2. It was convenient and rapid, high sensitivity and good selectivity and was applied to the determination of Th(Ⅳ) in marine products and catalysts with satisfactory results.