An athermalization design of switchable dual-field-of-view optical systems for 8～12 μm wavelength band using infrared focal-plane array (FPA) is presented. In order to switch the field-of-view (FOV), two different optical configurations are explored and compared. The first scheme is based on the axial motion of a lens group and the second is based on a rotate-in motion of two widely separated lens groups. It is found that the rotate-in scheme achieves better performance than the step-zoom scheme, but the optical passive athermalization misfits this scheme. The step-zoom scheme is easy to use the optical passive athermalization to achieve lighter weight and smaller volume, and the performance is well for small zoom ratio. Aspheric surfaces are utilized to control aberrations and reduce the total lens cost. The results show that the optical system in the 7.7～9.7 μm wavelength band has a compact structure and can achieve 100% cold shield efficiency. The final optical designs along with their modulation transfer function (MTF) are presented, showing excellent imaging performance in dual FOVs at the temperature range between -60 ℃ and 70 ℃.