Parameters about Z-scan measurements of thin films were investigated using numerical methods. Differences of the magnitude of Z-scan′s third-order nonlinear refractive index and absorption coefficient were observed under different laser pulse duration using numerical simulation. The more obvious peak-valley features of Z-scan′s closed-aperture curve were observed with the smaller aperture hole by simulating under aperture radius for different hole. The more obvious peak-valley features of Z-scan′s closed-aperture curve were observed by simulating under different distances of aperture lens to focus, when distance is 1/2 lens focal length. The experimental results reported in the literature related agree well with the theoretical analysis. The results have some significance to the measurement of Z-scan with films.