Abstract: Based on Fresnel-Kirchhoff diffraction theory, the propagation behavior of the Gaussian beam passing through the nonlinear optical medium is theoretically analyzed under strong absorption or without absorption. A unified equation is derived, which can describe the open-aperture Z-scan curve and the closed-aperture Z-scan curve. The numerical simulation of Z-scan curve under different conditions shows that the transmission Z-scan curve with peak and valley has some new features. As the incident laser peak intensity increases, the transmittance peak is suppressed and the valley is strengthened; increasing of nonlinear phase shift can heighten the peak and deepen the valley; the larger the laser beam waist radiusis, the more obvious the features of peak-valley configuration are.