Based on split-step fast Fourier transform method and coordinate transformation, we establish simulation methods of propagation and focusing of arbitrary beam shape in the light-induced refractive index change medium, in order to calculate Z-scan opening and close aperture fitted curves of arbitrary beam shape and medium thickness. Under the conditions of thin medium and Gaussian beam, the fitting curve coincides with the classical algorithm. In the case of flat-top beam, the fitting results of this method are consistent with the experimental data. By using this method, the nonlinear absorption and nonlinear index coefficient of LiTaO3 crystal are measured. Finally, the Z-scan process of Gaussian beam and vortex beam in thick media is analyzed.