Applications of Fourier transform spectroscopy in atmospheric science require high resolution, high accuracy measurement of spectrum to obtain the retrieval of physical quantities such as temperature, pressure and concentration. The instrument spectrum function of Fourier transform spectroscopy is influenced by the angular distribution of light on the interferometer' s off!axis detector.The effects are studied by direct calculating interference modulation function.Spectrum shape is simulated for rectangular detector based on the sounder' s 16×4 plane array detectors structure. The result can be applied in the spectral calibration for space!borne interferometers.