The conception of the High Etendue Imaging Fourier Transform Spectrometer (HEIFTS) is introduced and the principle of the modified Mach-Zehnder interferometer,which is used in the spectrometer,is analyzed. Meanwhile,the optical path difference and the degree of modulation of this kind of interferometer are calculated. It's easy to be found that,in order to get the better diagram of the degree of modulation,both the inclination of the two image planes and the image aperture angle must be small angles,and both the lateral and longitudinal offsets must be smaller. Then the effects on the degree of modulation by the offsets and the dimension of the CCD array are discussed. By adjusting the parameters,the three degrees of the modulation can reach 90%,which can satisfy the application requirement of the spectrometer.