Based on requirements of laser ranging and infrared target detection, a laser/ infrared compact optical system with a common aperture and athermalization is designed. The parameters of the optical system are defined as follows: the operating waveband covers 1.064.m of laser and 7.7-9.3.m of wavelength infrared radiation (LWIR), the diameters of the two entrance pupils are 120 mm; the focal length of the laser is 800 mm; the focal length of the LWIR is 240 mm, the F-number is 2, the field of view is 2.29°×1.83°. the vertical size of system is shortened by utilizing a catadioptric optical system with RC. The laser system shares the primary mirror and secondary mirror with the LWIR system. The beam splitting of the laser and LWIR is achieved by the secondary mirror. By using the double imaging configuration, the LWIR system exhibits nearly 100% efficiency of the cold stop. Optical passive athermalization is realized by choosing the appropriate optical materials, mechanical materials and assigning the optical power. In the temperature range of -50℃ to 70℃, the energy concentration of the laser receiving optical system is extremely high, and the image quality of the LWIR system is also quite good, which satisfies the requirements of practical applications.