Abstract: The improvement on the double-side band FFT is presented, we use the method of the single-side band FFT to transform the doubleside band interfero-gram.The spectra transformed by the improved method are compared with those by the double-side band FFT and Forman's method, the results showed that this improved method is satisfactory and its computing speed is faster than the others, and the noise of the spectra still keep linear and random.