The high spectral resolution Fourier transform spectrometer used in the aerospace field mostly adopt the scheme with a beam splitter and compensator for light splitting. However, for the machining error, the thickness between splitter and compensator cannot match perfectly, and the matching error wound impact the interferometer modulation value which is an important parameter of the instrument SNR (Signal Noise Ratio). For the requirement of high SNR, the matching error (ME) between splitter and compensator must be limited, which means thickness matching design. Firstly, the impact caused by the thickness matching under different situation was analyzed with theoretical analysis, and then the formulas of additional optical path difference (OPD) and signal intensity are given. Secondly, the interference signal caused by ME is visually displayed with Zemax simulation and it can be obviously observed that the modulation value decreases with the increase ME. Furtherly, the view field of the extended light source will cause oscillation in the incidence angle of non-central light of field, which can increase the variance range of OPD and enhance the influence on modulation with ME. Beyond that, for the wave number difference between the light source and metering laser, the dispersion effect will be introduced into the spectrometer system and the ME will enhance the influence on the signal with dispersion effect. For phase retrieval requirement, the ME must be limited to ensure that the dispersion phase difference is no more than 2π. Finally, based on the theoretical analysis, the design principle of the thickness matching between splitter and compensator was summarized, and it provided a design method for the tolerance analysis of the interferometer, which will improve the performance of the Fourier transform spectrometer.