Waste drug compounds can end up in the environment as pollutants in natural drinking water and municipal wastewater if not properly treated. Hospital sewage will contain low concentrations of drugs, and when these drugs enter the environment, they will become pollutants, which will seriously pollute the natural ecosystem. Indomethacin is a widely used non-steroidal anti-inflammatory drug, but it is not easily soluble in water, and refractory degradation makes drug degradation in sewage a challenge. In order to study the changes of the molecular structure and spectrum of indomethacin under the action of the external electric field (EEF), the density functional theory (DFT) and the 6-31+G(d, p) basis set are used along the Y-axis (N15-C16). The EEF (0～0.025 a.u.) was applied and the ground state geometry of the indomethacin molecule was optimized. The total energy, bond length, infrared spectrum (IR), dipole moment and HOMO-LUMO energy gap were investigated. The results show that in the absence of EEF, the single bond between C2 and C17 in the indomethacin molecule is optimized to become a double bond between the benzene rings, which makes the electrons of C16 and C17 and the isolated electrons of N15 form a strong bond with the benzene ring. The conjugated system minimizes the energy of indomethacin and forms the most stable configuration. The total energy of the ground state decreases slowly with the increase of EEF. When F≥0.015 a. u., it decreases significantly, and the change of dipole moment is opposite. As the EEF is enhanced, the expansion and contraction of each key length vary. The bond lengths of C3-C4, C3-N15, C5-C6, O10-C11 and N15-C16 are elongated, especially the bond lengths of O10-C11, C3-N15 and N15-C16 are drastically changed, and the easiest to break and then Indomethacin decomposition. When the EEF becomes larger, the energy gap is continuously reduced, indicating that the electrons of the indomethacin in the EEF are easily transitioned to a high energy level, and the molecules are excited to an excited state. The IR generated by the vibration of different chemical bonds in the indomethacin molecule, corresponding to different spectral shifts, is mainly related to the energy level, the energy level difference is reduced, the frequency is reduced, resulting in red shift (RS), and vice versa. Shift (BS); but the correspondence between N15-C40, C16-C18 bond length change ΔR and frequency shift change Δf indicates that the spectrum shift is also related to factors such as molecular orbital configuration and dipole moment change. Stronger 4, 5, 6, and 7 absorption peaks occur RS and the vibration intensity increases, indicating that the corresponding chemical bonds become weak and cause a fracture. All these phenomena indicate that indomethacin molecules become unstable and prone to dissociation with the enhancement of EEF. Analysis of the molecular structure and IR under EEF can be used to study the degradation of indomethacin by electric field dissociation method, in order to provide theoretical guidance for the degradation of stubborn drugs in sewage.