Various environmental poisons have caused damage to human production and life, and dioxin has seriously harmed human health. The C12H4Cl4O2(2, 3, 7, 8-tetrachlorodibenzo-p-dioxin, TCDD) is currently the most toxic compound. In order to study the influence of external electrical field on molecular structure and spectrum, herein the density functional theory (DFT) at a B3LYP/6-31+g (d,p) level is employed to calculate the geometrical parameters of the ground state of TCDD molecule under external electric fields ranging from 0 to 0.025 a.u. (0-1.2856×1010 V/m). Based on the optimized structure, time-dependent DFT at the same level as the above is adopted to calculate the absorption wavelengths and the molar absorption coefficients for the first twenty-six excited states of TCDD molecule under external electric fields. The results show that the most absorption band located at 221 nm with a molar absorption coefficient of 54064 L·mol-1·cm-1 in the UV-Vis absorption spectrum appears in the E belt, which originates from the benzene electronic transition from π to π*. In addition, a shoulder peak at 296 nm appears in the B belt, which is the characteristic absorption of aromatic compounds' electron transition from π to π*. Compared with the data in the literature, the wavelength of the shoulder is blue-shifted only 9 nm. The molecular geometry parameters are strongly dependent on the external field intensity, and the total energy decreases with external field intensity increasing. With the enhancement of external electric field, the electrons in the molecule have an overall transfer, which makes the big bond of benzene ring weakened, the energy of the transition decreases, and the wavelength of the transition increases, that is, the absorption peak is red-shifted. When the external electric field increases to 0.02 a.u., the electron cloud migration phenomenon of occupied and transition orbits of TCDD molecule are obvious, and the absorption peak red shift phenomenon is also very significant. With the enhancement of external electric field, the overall transfer of electrons in the molecule also reduces the density of the benzene rings and the surrounding electron cloud, reduces the number of electrons in the transition from π to π*, and also reduces the molar absorption coefficient. When the external electric field is enhanced to 0.02 a.u., the molar absorption coefficient decreases significantly. This work provides a theoretical basis for studying the TCDD detection and degradation method, and also has implications for other environmental pollutants detection methods and degradation mechanisms.