Abstract: With the advancements in infrared detector technology, long-wavelength infrared imaging lenses having a large relative aperture and large-array are being fabricated. In this study, a long-wavelength infrared athermalized lens with a large relative aperture (F/#=0.8) was designed for a large-array detector (1024 × 68@12 μm). Based on the temperature characteristics of different infrared materials and the theory of optical passive athermalization, the lens has a combination design of three infrared materials and an aberration correction design of four aspherical surfaces. The system is developed such that the spot diagram and modulation transfer function curve of every field of view have only small changes in the temperature range of −40–60℃, thus satisfying the optical non-heating design requirement. The lens has the advantages of a high luminous flux, compact structure, and good manufacturability. It can be used in driver assistance systems and aircraft pods for situation awareness.