Abstract: In order to realize the miniaturization of the laser/infrared dual-mode seeker imaging system and simplify the optical structure, a dual-mode common optical path annular aperture ultrathin imaging system with four reflections is designed. The design principle of the optical splitting path of the system is studied, and the relationship between the obscuration ratio and the field of view is given. The long-wave infrared (LWIR) 7.7-9.5 μm and laser 1.064 μm dual-mode seeker imaging system is realized by a single optical element. The focal length of the dual-mode annular aperture system in the LWIR is 70 mm, the equivalent F number is 1.3, and the full field of view is 8°. The modulation transfer function (MTF) values of each field of views are greater than 0.136 when the spatial frequency is 41.7 lp/mm. The focal length of the dual-mode annular aperture system at the laser wavelength is 53.8 mm, the equivalent F number is 1, the full field of view is 10°, and the spot diagram in the full field of view is uniform. When the ambient temperature ranges from −40 ℃ to 80 ℃, the MTF value of each field of view is greater than 0.13 in the LWIR, and the energy distribution is basically unchanged in laser wavelength. As a result, the optical passive athermalization is realized. According to the tolerance results, this system is machinable.