Abstract: In the course of spectrum reconstruction of Fourier-Transform spectrometer, there will be large error of the spectrum if the interferogram is transformed directly without any other process, because there is machining error as well as some other errors in the system. According to the theory of Sagnac FourierTransform spectrometer, the method which we used to reconstruct the spectrum correctly from the captured interferogram was developed. With the steps of trend term correction, apodisation, phase correction and conjugate symmetrization applied to the interferogram captured and then reconstructed it, the error was avoid.The experiment to reconstruct the spectrum of monochromatic light source such as Helium lamp used in the developed prototype was carried out. The results agree with the nominal spectrum distibutions of the sources with the accuracy of 4nm.