Taking the one-dimensional asymmetrical π conjugation molecule material （DBASVP molecules） as medium, with the two-photon resonance, we studied the propagation of ultrashort laser pulses in the medium based on the two-photon area theorem and the numerical solutions of the coupled Bloch and Maxwell's equations. The evolution of two-photon area is thus investigated and the applicability of the two-photon area is then discussed. A theoretical method for simulating the optical limiting properties of molecular medium is pointed out. The molecular electronic structures and dipole moments are calculated by use of density functional theory at ab initio level. The numerical results show that the two-photon area theorem based on the slowly varying envelope and phase approximations cannot accurately describe the evolvution of the two-photon area in the molecular medium. The optical limiting behavior based on the two-photon absorption is related to the width of the molecular medium.