The initial decomposition micro-mechanism of energetic materials has attracted much attention because it is a critical factor for the safe use of energetic materials. The thermally triggered chemical reactions are usually related to the vibrational properties of molecules. A time-resolved coherent anti-Stokes Raman scattering (CARS) spectrum system is constructed to study the molecular coherent vibrational dynamics of nitromethane at a microscopic level for clarifying the relation of molecular vibration to initial chemical reaction. In this experiment, the ultra-continuous white light is used as Stokes light, and the CARS spectra of different vibrational modes can be obtained by adjusting the time delay of the Stokes light. The vibrational dephasing time of different chemical bonds in nitromethane is provided by fitting the vibrational relaxation curves. The dephasing time of the CH stretching vibration located at 3000 cm-1 is shown to be 0.18 ps, which is far less than the dephasing time 6.2 ps of the CN stretching vibration located at 917 cm-1. The vibrational dephasing time is closely related to thermal collision for liquid nitromethane system without intermolecular hydrogen bond, that is, the scattering of thermal phonons causes the dephasing of coherent vibration. Therefore, the stretching vibration of the CH bond is more easily affected by the thermal phonon than the stretching vibration of the CN bond. The CH bond of nitromethane molecule is expected to be excited first, causing an initial chemical reaction under thermal loading.