Abstract: Taking the nonlinear refraction and the nonlinear absorption into account simultaneously, this paper investigated the diffraction theory model of the Z-scan technique, and analyzed the features of Z-scan curves and their variation rules. In the case of small nonlinear refraction and absorption the results obtained by this theory are consistent with that of the conventional Z-scan theory. For the materials with the large nonlinear refraction and absorption, the conventional theory become ineffective, but this theory can still give very good results. Numerical results show that the ratio of the nonlinear refraction to absorption is an important parameter that influences the optical power transmitted from the receiving aperture. To acquire a good optical limiting performance, the ratio of the