A semi-active laser and longwave infrared compound optical system with common aperture is introduced.The catadioptric system is used in the compound system,and the semi-active laser system and the infrared system achieve the common aperture at the primary mirror.According to the characteristic of no aberration for the mirror,optical transmission with common aperture and common optical path can be achieved effectively.After reflected by the mirrors,the two wavebands are split by the beam splitter,and then the split light enter into the corresponding detectors respectively.The compound optical system has the advantages of compact structure,no aberration and low cost.