In order to study the influence of external electrical field on molecular structure, chemical bond and electronic spectrum of environmental poison chlorophenol, the method B3LYP of the density functional theory (DFT) at 6-311++G(d, p) level is used to calculate geometrical parameters, dipole moments and total energies of the ground state of pentachlorophenol molecule under different external electric fields (from 0 to 0.025 a.u.) in this article. On this basis, the UV absorption spectra of pentachlorophenol (PCP) are studied using the time-dependent density functional theory (TDDFT) in the same fundamental group and compared with the ultraviolet absorption peak of phenol given in the literature. Finally, the rules of external electric field influencing wavelengths and oscillator strengths of the first ten excited states of a PCP molecule are studied. The results show that molecular geometry is strongly dependent on the field intensity, the molecular dipole moment is proved to be first decreasing, then increasing and the total energy first increasing then decreasing with the increase of the field intensity. Compared with the ultraviolet absorption peak of phenol, that of PCP is red-shifted. The oscillator strength of excited state of PCP is proved to be decreasing, and the ultraviolet absorption peak is also red-shifted with the increase of the field intensity.