To make a space-borne Fourier transform infrared spectrometer as an effective load on the FY 3 and FY4 satellites, the mechanical scanning methods of the Fourier Transform Spectrometers in the space-borne projects are compared and discussed, then a new method is put forward. It is based on the principle that when light passes through two parallel plane mirrors the optical path length depends on the incident angle. The new method correlates the optical path length changes between the two arms of the interferometer. This method requires only plane mirrors, and it is immune to tilt, wearing etc. Therefore this method is suitable for our project.