Infrared thermal microscope imaging system can be used to analyze the temperature changes of tiny objects or the local details, which has been widely used in industrial detection, medical diagnosis, scientific experimental research and other fields. The design method of the infrared zoom microscope optical system is analyzed, and an optical system with an imaging magnification of 0.5 ×~2 × is designed based on the refrigeration infrared detector. Mechanical compensation zoom is used in the system. The cooling diaphragm of the detector is matched with the pupil of the optical system by the secondary imaging configuration. The higher-order aberration is corrected by the two aspherical surfaces on eight lenses. And the smooth and continuous zoom in the full magnification range is realized. Through the analysis of zooming process by ZEMAX macro program, the optical system is designed with smooth movement, stable imaging and small change of dispersion spots during the whole zooming process.