The electronic excitations and electron absorption spectra of polychlorinated dibenzo-p-dioxins (PCDDs) were investigated by the time-dependent density functional theory (/DFT). The main spectral features were interpreted on the basis of the electronic structure of PCDDs by fully considering the possible interference of coexist organic molecules for the electron spectra. With the numbers of benzene rings in aromatic molecules increasing, the excitation energies decrease. The excitation energies of the compounds with three or four benzene rings (phenanthrene or fluoranthene) fall into the electronic transition regions of PCDDs. Therefore these compounds were not possible differentiate from the electron spectra of PCDDs. Furthermore, with the amounts of benzene rings extending to surpass three and four, the energies decease continuously and run beyond of the transition energy ranges of PCDDs. Thus the electron excitation energies of those aromatic molecules with three or four benzene rings were in the range of PCDDs, acting as the possible interferential substances for the detecting of PCDDs.