Based on Fresnel-Kirchhoff diffraction theory, a diffraction model of nonlinear optical media to Gaussian beam is set up, which can explain the Z-scan phenomenon from a new way. This theory is not only well consistent with the conventional Z-scan theory in the case of smaller nonlinear phase shift, but also can fit for the larger nonlinear phase shift. Numeric computations indicate the shape of the Z-scan curve is greatly affected by the value of the nonlinear phase shift. The symmetric dispersion-like Z-scan curve is only valid for small nonlinear phase shift (<π), but with increasing the nonlinear phase shift, the valley of the transmittance is severely suppressed and the peak is greatly enhanced. The laser power through the aperture will oscillate and attenuate as the nonlinear phase shift increases due to the increasing of the input laser power.