The C24H38O4 (dioctyl phthalate, DOP) is a main component of the plasticizer. In order to study the influence of external electrical field on molecular structure and spectrum of DOP, the method B3LYP of the density functional theory at B3LYP/6-311G(d,p) level is employed to calculate geometrical parameters of the ground state of DOP molecule under different external electric fields (from 0 to 0.0125 a.u.) in this article. On this basis, the ultraviolet-visible absorption spectrum of DOP is calculated by using the time-dependent density functional theory in the same fundamental group and compared with the ultraviolet absorption peak of the molecules, measured by UNICO ultraviolet and visible spectrophotometer. Finally, by using the time-dependent density functional theory in the same fundamental group, we study wavelengths and oscillator strengths of the first twenty-six excited states of DOP molecule in external electric field. The obtained results are as follows. The strongest absorption of ultraviolet-visible absorption spectrum appears in the end absorption band from n to σ* transition. The stronger absorption occurs in the E band of benzene electronic transition from π to π*. The molecular geometry parameters are strongly dependent on the external field intensity. The dipole moment of DOP molecule is proved to first decrease and then increase with the sharp increase of external field, but the total energy first increases and then decreases with the increase of the external field intensity. The ultraviolet absorption peaks of excited states of DOP are proved to have observably red shift, and the oscillator strength sharply decreases with the increasing of the field intensity.