Abstract: The qualification and function of image analyzer are analyzed, and the design process and results of optical system for the image analyzer in 8μm～12μm waveband OTF testing equipment are expounded according to the principle of the digital Fourier transfer analysis method for measuring the OTF. The scheme of the collimated beam path was adopted in this design, in which a catadioptric mirror was introduced into the beam path for compact system construction and for putting the filter in the parallel beam path to eliminate the transmissivity deviation coursed by the different incident angles and the effect of optical path difference introduced by different filters on the image plane location. The object NA of the system is 0.85. The perpendicular axis magnification ratio is 1.5× and the maximum aperture is Φ25mm. The system is verified by the experiments.