Colorimetric method is used to study the decomposition of CL-20 dissolved in concentrated sulfuric acid. The result show that the decomposed amount of CL-20 in a certain time is inversely proportional to the concentration of sulfuric acid. The decomposed amount of CL-20 in 10 ml 98% sulfuric acid in 2 h is at most 0.02 g.The content of water in the nitrating mixture should be diminished as much as possible in order to improve yield and guarantee safety in CL-20 production.