In order to analyze the influence of external electric field on the molecular spectra and dissociation of methyl chloride, density functional (DFT) theory at B3LYP/6-311G++(d,p) level is used to investigate the influence on the molecular structure of the chloromethane molecule and dissociation properties under different external electric field (－0.03～0.03 a.u.), including molecular bond length, energy gap, IR spectra, dissociation curve and so on. The calculation results show that with external electric field increase from －0.03 a.u. to 0.03 a.u. step by step, on the Z direction(C-Cl line), C-Cl bond length increases and C-H bond length is almost the same. When the external electric field is equal to 0.01 a.u., the peak value is reached. The molecular energy gap increases and then decreases. The C-Cl stretching (STR) vibration frequency gradually decreases, while infrared (IR) vibration intensity gradually increases. Further research and analysis indicate that with the continuous increase of the external electric field (0～0.03 a.u.), the energy curve of CH3Cl molecule decreases gradually and the dissociation decreases too. When the external electric field is about 0.04 a.u., the dissociation energy is at the minimum, and the dissociation occurs. Therefore, it can control the degradation of the CH3Cl molecule by changing the external electric field.