A design study of athermalization of switchable dual-field-of-view optical systems for 7.7~9.7 μm is presented. With the axial motion of a lens group, a result of dual-field-of-view infrared optical system is achieved. In order to meet a wide environmental temperature range, the optical passive athermalization is used to eliminate the thermal effect in optical systems by correctly assembling different optical materials. The final optical designs along with their modulation transfer function (MTF) and root-mean-square (RMS) radius are presented, showing excellent imaging performance in dual fields-of-view at the temperature range between -60 ℃ and +70 ℃.