The computer simulation for spectrum diffraction interferometer (S-D-I) is proposed. The S-D-I with spectrum of geometric pattern is discussed, and the intensity is directly relative to the line-integral along the track of the pattern. Acording to the experiment, we proposed a parabola model of an alcohol-lamp flame, thus the distribution of interference fringe of the flame can be calculated with a computer, and the result is identical with the experiment. We also discuss the nature of S-D-I with some special spectra, and a few valuable conclusions are obtained.