The two-photon absorption properties of 4,4′-bis (dimethylamino) stilbene synthesized recently are calculated at ab initio level. The hybrid density functional theory is embraced to optimize the geometry of the molecule and the time-dependent density functional theory is used to study the nonlinear optical properties of the molecule. Simulation of the solvent effects is carried out by the self-consistent reaction field theory with the polarizable continuum model. The numerical results show that three-state model is good enough to give the two-photon absorption cross section for the lowest excited states, which is consistent with experimental measurement quite well. As the increase of the polarity of solvent, the wavelength of single photon is red-shifted and the two-photon absorption cross section is increased slightly. The TPA cross sections agree well with the experimental data.