In order to understand the effect of polymorphic transformation on the impact sensitivity of heated hexanitrohexaazaisowurtzitane (CL-20) and its composite, the polymorphic transition of CL-20 crystal under heat stimulation was investigated using powder X-ray diffraction (PXRD) and microscopic methods. The effects of particle size and PDA coating on polymorphic transition of CL-20 were explored. The experiments showed that the polymorphic transition at 160 degrees C was related to the particle size and surroundings of CL-20 crystals. As the temperature increases, changes in the molecule conformation of larger crystals were more facilitated than in smaller ones, the epsilon ->gamma solid-solid phase transitions went along thoroughly in larger crystals. Poly (dopamine) (PDA) coating played a key role in the significant inhibition of phase transition owing to the restriction effect via its compact and rigid shell. The lower pressure caused by drop-weight impact can facilitate the epsilon ->gamma phase transition of heated CL-20. This investigation contributes to comprehend the polymorphic transformation mechanism of CL-20 and provides guidance for effective control on the CL-20 polymorphs in CL-20-based composites.