IR optical system of roll-elevation seeker was composed of object lens front group, turning mirrors group and imaging rear group. The pitch frame had ±90° moving range and the roll frame had 360° moving range, so that the combination of rolling and pitching enabled the optical system to realize all over the front hemisphere field. The optical system achieved 100% cold shield efficiency. In order to adapt to the large variation of working environment temperature, the athermal design was used, and the formulas to realize athermal design were given. By using the technique named dummy chromatic aberration, the best initial solution of athermal design was found quickly. For controlling the Narcissus effect of this cooled IR optical system, the formula to compute the NITD were given, and the sensitive surface of Narcissus effect was optimized. The design result showed that the optical system working parameters were the effective focal length of 58 mm, field of view of 4.0°, F/# of 2.0, and the MTF of the optical system was close to the diffraction limit when working temperature changed from -50 ℃ to 60 ℃. The narcissus effect was also controlled effectively. The optical system met the design requirements by imaging test.