The polymorphic transformation （PT） and control technology of hexanitrohexaazaisowurtzitane （CL‍-20） has been a hot area of research in energetic materials， which is also the key issue must be addressed to promote its application. In order to further understand the PT characteristics and mechanism of ε‍-CL‍-20 with different crystallization characteristics， the PT laws and isothermal PT kinetics of ε‍-CL‍-20 were studied based on in‍-situ X‍-ray powder diffraction （XRD） technology. The effect of surface and internal defects on the ε→γ PT behavior of CL‍-20 was discussed. The isothermal PT kinetics of ε‍-CL‍-20 with different crystallization characteristics was analyzed and the related parameters were calculated. The results show that temperature is the dominant factor affecting the solid‍-solid PT of ε‍-CL‍-20. For the conventional particle ε‍-CL‍-20， with the increase of internal and surface defects in the crystal， the initial temperature of PT decreases and the PT rate increases. Compared with 100 μm CL‍-20， ultrafine （0.5-1 μm） ε‍-CL‍-20 has higher starting temperature of PT， but its PT rate is also faster. The abnormal PT behavior of ultrafine ε‍-CL‍-20 is explained from the two‍-sidedness of crystal defects. When CL‍-20 undergoes ε→γ PT under thermal stimulation， both the surface and internal defects of the crystal have an induction effect on the PT process， and the γ crystal preferentially nucleates at the defects such as vacancies， impurities or dislocations with low nucleation barrier on the ε‍-CL‍-20 crystal， and then gradually grows up at these positions.