Thorium is one of the scattered elements in coal, is a kind of radioactive element, may pollute the environment and endanger human health, so it is very important to the analysis of thorium.The trace element thorium in coal samples was measured by sodium peroxide fusion, hot water extraction, filtration,dissolution of precipitation using 4 mol / L hydrochloric acid in a volumetric flask,coloration of solution using arsenazo III under the masking agent and with hydrochloric acid as medium, and then determination of absorbance at a wavelength of 650 nm using the spectrophotometer,determination of the content of the element thorium according to the absorbance value and through the work curve. The results showed that the method of determination of thorium content in coal was rapid, simple, a common analysis method for determination of thorium.