There is many interference information of targets contained in the interference fringes which are obtained by the variable gap Fabry-Perot (F-P) interferometric imaging system. The sinusoidal nature and modulation of the interference fringes directly affect the extraction of interference information of the target. The reflectivity value of the variable gap Fabry-Perot (F-P) interference cavity material is an important performance parameter of the variable gap F-P interferometric imaging system. In order to improve the performance of the variable gap F-P interferometric imaging system, the reflectivity of the variable gap F-P interferometric cavity depends on the sinusoidal nature and the modulation of the interference fringes. In this paper, the mathematical relationship between the sinusoidal nature and modulation of interference fringes and the reflectivity of F-P cavity with variable gap was established respectively and a reasonable objective function was given. When the sinusoidal nature and modulation of the interference fringes were relatively optimal at the same time, the optimal reflectivity of material of the F-P cavity with variable gap was found. The long wave infrared variable gap F-P interference spectral imaging system was built, and the correctness of the above theoretical analysis was verified by measuring the spectral transmittance of polypropylene film.