Based on the requirements of double-band detection MIR and LIR wave RC optical system with common aperture is designed. The cold stop match of the detector is achieved to reduce inner stray radiation, system volume and weight with secondary imaging mode. The focal length of the common aperture system is 400 mm. Modulated transfer function (MTF) is 0.45 at 33 lp/mm middle wave IR detector cut-off frequencies. MTF is 0.4 at 20 lp/mm long wave IR detector cut-off frequencies. The system has good performance.