Infrared search and track (IRST) system with the characteristics of long detection range, good camouflage and low error rate becomes the best detection equipments in the air and on the sea. According to the multispectral demand of IRST system, an IRST optical system with dual-band common aperture is designed. The system is better than the single-band system in the area of acquiring information. The obtained MWIR and LWIR images can be fused together so as to take the advantages of MWIR and LWIR synthetically. Effective reconnaissance probability is enhanced and false alarm probability is reduced. Total focal length of the system is 400 mm, F#=2 and field of view is 2°. The clear imaging in 3~5 μm and 8~12 μm with dual-band common aperture is realized by dispersive RC system. Second imaging is performed on a medium wave system for suppressing thermal radiation stray light from medium wave. Design result indicates that excellent quality images can be produced by the system and the operation requirements of IRST system can be met.