The one_photon and two_photon absorption properties of a series of 1，4_di methoxy_2，5_divinyl_benzene derivatives are investigated by use of the analytic response theory at HF level. The one_photon and two_photon fluorescence spectra of E，E_2_5_Bis［4′_(N,N_di_n_butylamino) styryl］_1，4_dimenthoxybenzene are measured. The numerical results show that these molecules have strong two_photon absorption cross sections. In the visible light region，the maximal one_photon absorption strengths of the molecules occur in the first excited state. Furtherm ore，the maximal two_photon absorption cross sections of the D_π_A typed molecu les still appear in the first state，　but for D_π_D typed molecules the second excited state has the maximal two_photon absorption cross_section.　The optical properties of the molecules are closely related to the electric property of the donor and acceptor.　The symmetrically substituted molecules can not be determi ned to have a larger two_photon absorption cross section than the asyrnmetricall y substituted molecules for such a kind of π_center.　The charge_transfer proce ss for the charge_transfer states is displayed，and then the mechanism of photop olymerization is discussed qualitatively.