With the demand of autonomous precision guidance of air defense missile, the system scheme of the IR imaging/Ladar dual-mode seeker with a common aperture was proposed, and the optical system used in was designed. The system had a common receiving aperture, and its structure was very compact, so it could meet the requirement for the miniaturization of the seeker. Besides, it also could meet the demands of a wide field of view for searching target, and the demands for accurately recognizing and tracking the target at the same time. In order to increase the narrow FOV tracking performance, the dual FOV infrared optical used the zooming mode which some components flip in or out the optical system to firm the target signal. The dual FOV optics are divided into the zooming part, with dual variable focal length, and the reimaging part which was chosen in such a way to minimize the objective lens while maintaining 100% cold shield efficiency. The final infrared optics including 4°×3°(NFOV) and 16°×12°(WFOV) was designed. The NFOV lens composed of two common IR/Ladar lens, three relay lens, a beam splitter and two reflective fold mirrors, while WFOV lens increased two lens such as Germanium and Silicon. The common IR/Ladar lens ZnS and ZnSe could refractive the IR optics and Laser optics. The beam splitter which refractived IR optics and reflected Laser optics was located in the middle of Germanium and Silicon. The designed optical system had good image quality, and fulfilled the performance requirement of seeker system.