The Space-borne Fourier Transform Spectrometer (SBFTS) carried by FY-4 meteorology satellite is one of infrared remote sensing instruments.It acquires the temperature,pressure and humidity of atmosphere on geostationary orbit,and supplies the valuable weather data for next numerical weather prediction.For the proper use of the SBFTS,it is imperative to provide high quality spectral Calibration.The Instrument Line Shape (ILS) of Fourier transform spectroscopy is influenced by the angular distribution of light in the interferometer’s off-axis detector.Analyze the effect on spectral calibration by off-axis detectors,and then make ILS simulation for rectangular detector based on the sounder’s 16×4 plane array detectors structure.The result is used in the spectral calibration of the SBFTS.The precision of 10^-5 is achieved in the spectral calibration of SBFTS.Because of using focal plane array in Fourier transform spectrometer,the spectrum measured by the off-axis detector appears to shift to low frequency.The ILS of SBFTS must be analyzed for removing the effect of frequency shift in spectral calibration.