Due to contacts with additives or changes of environmental conditions （temperature or pressure）， the polymorphism hexanitrohexaazaisowurtzitane （CL-20） is easy to transform into mixed crystal form in propellant system， which leads to structural damage and performance degradation of the propellant. In order to hinder the contact between the solvent and CL-20， then inhibit the crystal transformation of CL-20， the polyphenol amine （PCHA） film was prepared based on the oxidative self-polymerization of hexamethylenediamine （HMDA） and catechol （CCh）. The surface of CL-20 crystal was modified by water suspension method under mild conditions. The scanning electron microscopy （SEM）， Fourier transform infrared spectroscopy （FT-IR）， Raman spectroscopy （RAMAN）， X-ray diffraction （XRD） and X-ray photoelectron spectroscopy （XPS） were used to study the morphology， coating content， thermal properties， and stability in ethylene glycol solution for the composite particles. The results show that HMDA and CCh can modify the surface of CL-20 crystal under mild conditions and form a dense PCHA coating layer. The content of PCHA is about 1% measured by dissolution weighing method and high performance liquid chromatography （HPLC）. The PCHA coating layer increases the crystal transition and thermal decomposition temperature by 16 ℃ and 7 ℃， respectively. The thermal decomposition activation energy E_a at different heating rates was calculated by Kissinger method. The activation energy of CL-20@PCHA is about 8 kJ·mol^-1 higher than that of CL-20， and the thermal stability is greatly improved. The XRD test results indicate that the PCHA film can effectively prevent the contact between the solvent and CL-20， slow down the dissolution rate of CL-20 in the solvent， and effectively inhibit the crystal transformation of CL-20.