Abstract: A method to design the hybrid athermal infrared optical system is studied. A design concept of the mutual compensation of the thermal dispersion and chromatic dispersion was introduced in the design, in which the equations that the athermal design should meet are acted as a constraint condition in the optimization. A refractive/diffractive hybrid athermal infrared optical system, whose cold shield efficiency is 100%, F/number is 2, field of view is 5, effectivefocal length is 70mm and working wavelength is 3.7m～4.8m, is designed. The image quality of the system approaches the diffraction limit at the working temperature of -40℃～60℃. The system can be compatible with the high-resolution cooled staring focal plane detector whose pixel size is 15m.