To meet the imaging demands of an airborne equipment with an ultra-wide field of view, a wide-angle, low-distortion, large relative aperture infrared optical system operating in the wavelength range of 3μm to 5 μm, and a field of view of 97°×97° is designed. A reimaging structure is adopted to reduce the diameter of the rear lens group, and on the basis of passive athermal design, the thermal expansion coefficient of the structural material is utilized to compensate for the residual thermal difference, achieving an athermal design over the entire temperature range. Distortion correction is achieved using "non-similar" imaging technology. The design results show that the system′s modulation transfer function (MTF) values are all above 0.4 at a typical spatial frequency of 33lp/mm, approaching the diffraction limit. The imaging quality is fine, with a maximum distortion of less than 0.6%, meeting the design demands.