Abstract: ThO₂ can produce many nuclides after accelerator bombardment, such as Actinium-225, Radium-223, Molybdenum-99, Iodine-131, Strontium-90, etc. There are many methods for measuring heavy elements such as thorium, but most of them are complicated to operate. Spectrophotometry has the advantages of simple sample preparation and low operation difficulty, and can be used as a quantitative analysis method for thorium and other heavy elements. ThO₂ was dissolved with nitric acid and hydrogen fluoride, and the concentration of Th(IV) in ThO₂ solution was quantitatively analyzed with Arsenazo III as the chromogenic agent. Under the environment of 2 mol/L hydrochloric acid solution and 0.1 g/L Arsenazo III solution, the coordination ratio of Arsenazo III and Th(IV) is 5 : 2 under the experimental conditions. The maximum absorption wavelength before and after color development is 534.0 nm and 663.2 nm, respectively, and the contrast is 129.2 nm. The method in 0.2-5.0 mg/L is in accordance with Lambert-Beer’s law, the standard curve equation y＝0.06641x+0.00176, R²＝0.99981. The molar absorption coefficient of 1.64×10⁴. Method detection limit is 0.31 μg/L, and quantitative limit is 0.97 μg/L. The method is stable within 4 h. The recovery is 97.33%-104.03%, and the largest standard deviation is 0.32%. The method was compared with GB/T 12690.12-2003, the process was simplified and the results were basically consistent.