In order to explore the thermal decomposition behavior of 3，5-dimethyl-4-hydroxyphenylpentazole， UV-visible light absorption spectrum combined with quantum chemical calculation was adopted. The thermal decomposition of 3，5-dimethyl-4-hydroxyphenylpentazole （HMPP） with increasing temperature was tracked. The thermal stability of HMPP was tested by differential scanning calorimeter. The results show that the initial decomposition temperature of HMPP is -14 ℃， and the characteristic absorption peak produced by the pentazole ring is at 284 nm in the UV-visible region. The characteristic absorption peak of 3，5-dimethyl-4-hydroxyphenyl azide （HMPA） is 258 nm. With the increase of temperature， the absorption peak of the whole system decreases gradually at 284 nm. In order to explore the specific reasons for the change of HMPP ultraviolet-visible light absorption spectrum， the decomposition products of HMPP were separated by column chromatography. The main decomposition products of HMPP were 2，6-dimethyl-p-benzoquinone and 4-（4-hydroxy-3，5-dimethylphenyl） amino）-2，6-dimethylcyclohexa-2，5-diene-1-one. After the thermal decomposition of HMPP， the uV-visible absorption of the whole system was determined to be caused by the superposition of uV-visible absorption of compounds 4 and 5.