The spectral information of interferogram is obtained by Fourier transforms with spatial heterodyne spectrometer (SHS). So the spectrum signal to noise ratio and spectrum quality are directly influenced by interferogram modulation efficiency. In view of the influence factors of interference fringe modulation is carried on the theory of quantitative analysis, including splitting characteristics of beam splitter, aberrations of optical system (surface wavefront error of interferometer elements, output wavefront error of collimating lens and modulation transfer function curve of imaging lens), the extended source and pixel size of detector. Experimental result show that the measured result of recovering spectra by the SHS instrument is in agreement with the theoretical calculation. Quantitative analysis of the interference fringe modulation for interferometric spectrometer provides theory basis for the design of each function module.