The principle and optical design method of cooled off-axis reflective optical systems were analyzed. An optical system design of a cooled off-axis three-mirror optical systems based on freeform surfaces was presented. This cooled off-axis three-mirror optical system was composed of two freeform surfaces and one even aspherical surface, which was a re-imaging structure with cold stop as the optical stop that could ensure 100% cold shield efficiency in the optical system. The optical stop could be imaged at the position of the first mirror by the second mirror and third mirror, which could reduce the size of the first mirror obviously. With the three mirrors optimized after optimization design including tilted and decentered methods, the system unobscured could be achieved and the freeform surfaces could be used to enlarge the field of view, correct image aberrations and improve the total image quality of the system. This system is with 450 mm focal length, 3-5 μm working waveband, 2 F-number, 3.662°×2.931° field-of-view. In addition, the modulation transfer function(MTF) of the system is above 0.5 all over the field of view with the working temperature range from -40 ℃ to 60 ℃. The result shows that the system has compact structure and athermalization achievement.