Phonon spectra play an important role in studying the thermodynamic properties of solids and the microscopic process of initiating chemical decomposition reactions， which can help to reveal the microscopic physical mechanism of initial thermal decomposition mechanism， detonation performance and sensitivity. In this work， the density functional theory with dispersion correction was used to calculate the phonon spectra and thermodynamic properties of 2，4，6，8，10，12-hexanitrohexaazaiso-wurtzitane（CL-20）/1，4-dinitroimidazole（1，4-DNI） cocrystal and co-formers. Through analyzing the phonon density of states， the way in which the phonon mode stores and transfers energy was determined， the direction of thermal energy flow was proposed， and the trigger bond and impact sensitivity order were predicted. The results show that the initial bonds of ε-CL-20 and CL-20/1，4-DNI cocrystal are predicted to be N─NO₂ bonds on CL-20 molecules； the initial thermal decomposition of 1，4-DNI may be related to the ring-opening of imidazole. By comparing the phonon density of states of CL-20 and 1，4-DNI molecules in cocrystal and its pure components. It can be found that the thermal stability of both CL-20 and 1，4-DNI molecules were improved in cocrystal， so that the thermal stability of the cocrystal being superior to the co-formers. According to the "doorway" mode phonon number and characteristic vibration frequency Δω_d， the order of the impact sensitivity is predicted of to be ε-CL-20>CL-20/1，4-DNI>1，4-DNI， completely consistent with the experimental measurement results. The thermodynamic parameters of CL-20/1，4-DNI cocrystal and co-formers have been calculated by phonon spectra， at the same temperature， the order is CL-20/1，4-DNI>ε-CL-20>1，4-DNI. In addition， low-frequency phonons contribute the most to heat capacity（C_V）， and the chemical bond breakage caused by energy transfer may undergo a multi-phonon up-pumping process.