In this paper, the process of degradation of polyethylene terephthalate (PET) by KOH as depolymerization agent under mild conditions was studied, and the reaction conditions of PET chemical degradation were optimized. The results showed that the degradation efficiency could be significantly improved by using KOH as catalyst and methanol as cosolvent. The optimum molar ratio of KOH to methanol was 1 / 9.2, and the hydrolysis rate of PET reached 96.6 % at 120 °C under normal pressure. At the same time, methanol can be directly distilled and reused at this temperature. Secondly, the product was quantitatively analyzed by HPLC analysis, and the reaction mechanism of PET chemical degradation was studied by IR and other means. In the reaction process, hydroxide ions attack the carbon positive ions on the long chain of PET to form tetrahedral anion intermediates. The intermediates are unstable and generate conjugated bases of carboxylic acids and alcohols. Carboxylic acids and bases are neutralized in an alkaline environment, and the final product is alcohols and carboxylates.