2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexazazoisowoodane (HNIW, CL-20) is a high-energy com- pound with a cage structure, which has broad application prospect in military and civil fields. In order to further understand the physical mechanism of the phase transition of CL-20 crystal, the absorption spectra in the temperature range of 23.0-179.8 ℃ are studied by using the terahertz time-domain spectroscopy (THz-TDS). The significant change of terahertz spectrum under thermal action indicated that the irreversible phase transition of CL-20 begins at 136.8 ℃. Combined with the results of solid-state density functional theory (DFT), the transition is identified as ε→γ-CL-20. Moreover, the analysis of the low-frequency vibration characteristics of CL-20 shows that the vibration modes of the molecular cage skeleton change significantly during the phase transition. The extensive van der Waals interactions between molecules are the source of this change. In addition, the evolution of the rotational vibration of the nitro group outside the framework is closely related to the molecular hydrogen bonding. This study provides a reference for further understanding the complex physical mechanism of CL-20 phase transition and detonation/deflagration under temperature loading. It is of great significance for the design and synthesis of high-quality explosives based on CL-20.