Para-xylene(PX) is an important raw material in chemical industry. In order to study the change of ultraviolet-visible(UV-Vis) spectra of PX under external electrical field, the density functional theory (DFT) has been employed to calculate geometrical parameters of the ground state of PX molecule under different external electric fields ( from 0 to 0.025 a.u.) in this paper. On this basis, the UV-Vis absorption spectra of PX molecule were calculated by employing the time-dependent density functional theory (TDDFT). At last, by employing the TDDFT in the same fundamental group, we studied wavelength and the molar absorption coefficient of the first twenty-six excited states of PX molecule under different external electric fields. The results show that the most absorption of UV-Vis absorption spectra appears in the E1 belt of benzene electronic transition from π to π*; the ultraviolet absorption peaks of excited states of PX are proved to appear observably red shift and the molar absorption coefficient sharply decreases with the increase of the field intensity.