In the paper,Fourier-transform method was introduce and was used for the design of infrared films on ZnSe substrate for 3~12μm. The refractive-index profile of Gradient-index film was obtained by inverse Fourier transformation of the desired spectral transmission curve and the corresponding spectral transmission curve was evaluated by using conventional matrix multiplication. The curve was refined through successive approximation and restructured new Q function to reduce the deviations from desired transmission curve. The quintic matching layers were overlayed at both interface of air and substrate. The average transmission of the Gradientindex film in the design wavelength range is 95%.