Abstract: The image quality of re-imaging infrared optical system is quite sensitive to temperature, compared with common optical system,its characteristics and effect was discussesed in the paper. And the optical-passive design theory of athermalization was described. An instance of a cooled re-imaging infrared optical system was given, the heating contrast experiment was carried out and the test result was shown. The athermal ability of the designed optical system and the common non-athermal optical system were tested by experiment with the same optical parameters such as focal length, field of views etc. The design result shows that the system is athermalized because the image quality of the system is close to the diffraction limit in the temperature range -40 ℃~+80 ℃ without focusing mechanism, and the shift rate of the focal length is relatively low. The test result shows the athermal ability of this system can remain its image quality in high temperature environment without refocusing.