The design of mechanically athermalized longwave infrared optical systems with wide field of view and large relative aperture is studied. An optical system with the F number of 0.8 is designed for the waveband of 8 to 12 μm. It is athermalized in the temperature range -40℃ to 60℃. The analysis results show that the optical system is simple in structure and good in imaging quality. It has an optical transfer function greater than 0.5 at the spatial frequency of 20 lp/mm.