Abstract: The optical path difference is an important character in Fourier transform spectrometer, and the optical path difference is produced by means of moving mirrors in spectrometer. Based on the scanning interference spectroscopy principle, the optical path difference of the spectrometer was analyzed theoretically ,and its calculation expression was given .Results show that the optical path difference is approximately proportional to the swing angle.Additional , the optical path differences between beams of light associated with different incident angles were also pointed out. The studies pave the way for optimization design of Fourier transform spectrometer.