An athermal optical design of a refractive/diffractive static infrared earth sensor, which meets the requirements such as portability, cheapness and reliability, is presented. The optical system has a wide-angle f-θ structure with a diffractive element introduced, which is beneficial to the correction of the monochromatic, chromatic and athermal aberrations. The inverse telephoto structure is adopted to achieve enough back-working distance. It is shown for the designed optical system that the f-θ linearity is less than 0.5% at 20 ℃, the field of view is 140°, the modulation transfer functions are greater than 0.52 at the spatial frequency of 20 lp/mm in the temperature range from -20 ℃ to 40 ℃ and the wave-front aberration is less than λ/4 for all situations, with a satisfactory back-working distance of 15 mm. These meet the requirements in the actual applications.