Polychlorinated biphenyls (PCBs) are persistent organic pollutant, and 2, 2, 5, 5-tetrachlorobiphenyl is generally used as a model molecule of PCBs in some studies. PCB52 molecule is degraded under external electric fields. The molecular structure of PCB ground state is optimized by density functional theory (B3LYP) method with 6-311+g(d) basis sets. The effects of electric fields ranging from-0.04 a.u. to 0.04 a.u. are investigated on structural parameters, total energy, dipole moment and charges distribution. The transition wavelengths, oscillator strengths and excitation energies of the first six excited states under external electric fields are calculated by the time dependent density functional theory method. The result shows that the bond lengths of 1C21Cl and 14C20Cl increase with external electric field increasing. The dihedral angle of two benzene rings of PCB52 molecule increases under the electric fields, and the PCB52 molecule reduces toxicity. PCB52 molecule energy gaps decrease, leading to the fact that the molecule is susceptible to excitation to an excited state and reductive dechlorination reaction. As the increase of the applied electric field, the excitation energies rapidly decrease, absorption wavelengths are red-shifted toward longer wavelength and oscillator strength is no longer zero, which indicates that the PCB52 molecule is easily excited and dissociated.