A new method with in-line F-T computer-generated holograms is proposed to synthesize the coded gratings for the coded grating F-T diffraction interferometry, so that it becomes effective in practice that various forms of interference can be achieved by using different masks. As a result, any abnormal forms of interference can be easily obtained. The required F-T spectrum commonly consists of geometric patterns, thus, another new idea is proposed that the corresponding object function can be expressed by pure phase variations in geometric consideration. Therefore, the CGHs used for coded gratings are really computer-generated interferograms that can be more easily prepared, and the calculation for 2D Fourier transformation can be totally economized.