The method of in situ X-ray powder diffraction (in-situ XRD) was used to investigate the effect of additives such as HTPB(hydroxyl-terminated polybutadiene), TDI(toluene-2, 4-diisocyanate), DOS(dioctyl sebacate), T-12(dibutyltin dilaurate), AP(ammonium perchorate) and Al(Aluminum powder) on the polymorphic transformation of hexanitro-hexaazatetracyclo-dodecane(ε-CL-20). Results show that, under heating condition, the preliminary polymorphic transformation temperature of ε-CL-20 changes by adding the additives. The crystal of CL-20 is coated to some degree by HTPB and TDI. As an adhesive catalyst, T-12 accelerates coating combined with HTPB and TDI. But in above two cases the transformation of ε→γ is inhibited. DOS enhances the transformation of ε→γ mixed with CL-20. In addition, AP/CL-20 and Al/CL-20 are both solid-solid mixing, which has little effect on the polymorphic transformation of ε-CL-20. Moreover, while heated at 70 ℃ for 60 h, the polymorph of ε-CL-20 in all the composite systems is not changed.