The combination of semi-active laser detection guidance and infrared imaging guidance can obtain active and passive detection modes, improving the seeker’s all-weather detection ability, anti-interference ability and target capture tracking ability. The performance of the optical system in the laser infrared composite guidance seekers directly determines the detection range, target recognition and hit accuracy of the guided weapon system. The composite seeker adopts a separate aperture optical system, and each channel adopts a separate optical structure and detector, which is easier to achieve, but it will bring problems such as spatial registration, and the system volume and mass are large. In this paper, a common aperture optical structure is adopted, and the transmittance of laser (1.064 μm) and infrared (8~12 μm) spectral bands is considered, and the composite optical design is achieved by using a focal length matching method to share a fairing and a high transmittance lens. The laser signal is focused through the fairing and the first lens, which is then reflected to the side of the lens barrel by a planar mirror located in the center of the aperture, which is received by a four-quadrant detector. The infrared light signal passes through the periphery of the aperture and is imaged onto an uncooled focal plane detector by three lenses containing diffractive lenses. The whole optical system through -45~60 ℃ no heating design, the overall structure of the optical system is simple and compact, small size, the total length is less than 86 mm, the caliber is less than 62 mm. The optical system design can be applied to laser infrared composite guided rockets, missiles and other weapon platforms, providing technical reserves for high-tech warfare with high efficiency and cost.