Using density functional theory and ZINDO methods, the structures, molecular orbitals and electronic spectra of a series of spirobifluorenes were investigated, in which two identical push-pull type chromophores are connected via an insulating carbon. Based on the correct electronic spectra, according to the sum-over-states formula, the calculation program was devised and nonlinear third-order optical susceptibilities were calculated. The results show a good agreement between the calculation and experiment. The calculated nonlinear third-order susceptibility γ of the spirobifluorenes are about six times as large as those of the corresponding mono-fluorenes without an increase of λmax. Since electronic coupling between monomer units in the spirobifluorenes is negligible owing to symmetry, we attribute the large enhancement to the spiroconjugation. Thus, we conclude that derived spirobifluorenes will be a hopeful type of third-order nonlinear optical material from the standpoint of the high transparency and relatively large γ value.