Abstract: A three-mirror anastigmatic(TMA) optical system was adopted to prevent chromatic aberration of the refractive system by adding a dichroic beam splitter behind the tertiary mirror to simultaneously implement the image to the MWIR and LWIR detectors. The integrated system included three off-axis mirrors and a dichroic beams plitter. The surface of the tertiary mirror was an XY polynomial freeform surface that could correct system aberrations. The structure of the system was re-imaged with 100% cold shield efficiency. The F-number was 2.67, the full field of view(FOV) was 11.4°×1.8°, the working band is 3.55-3.93 μm for the MWIR channel and 10.3-12.5 μm for the LWIR channel. The modulation transfer function (MTF) average values of MWIR were greater than 0.5 at 25 lp/mm, and the MTF average values of the LWIR were greater than 0.4 at 12.5 lp/mm. The temperature compensation of the optical system was optical passive athermalization. The temperature range was -40℃ to +60℃.