Abstract: A common-aperture catadioptric optical system for uncooled long-wave infared (LWIR) and semi-active laser compound guidance was designed. In order to reduce the difficulty of parts manufacturing and assembly of the reflective system, the secondary mirror of Cassegrain system was simplified to a plane mirror, and the primary mirror adopted metal paraboloid.The lens size of eyepiece group was optimized to avoid internal occlusion of optical path, and the optical passive athermalization design of infrared channel was realized by using the reflection system primary image plane and infrared materials. What's more, the system spectral splitting efficiency and transmittance were improved by setting the beam splitting plate and laser narrow-band filter in the parallel optical path. Final design results show that at the characteristic frequency of infrared channel, the modulation transfer function (MTF) >0.2, the laser linear region is 2°, which meets the system index requirements.