Abstract: For the Ka-band passive millimeter-wave(PMMW) imaging application, the antenna system design consisting of a dielectric lens and a feed antenna is completed. Combining Gaussian beam theory with the equal optical path principle in optical lens, a large aperture dielectric lens is designed, which has excellent focusing performance. A novel antipodal linear tapered slot antenna is proposed as the feed of the lens. The antenna exhibits excellent cross-section and high gain, and can form a dense feed array on the focal plane. The designed dielectric lens and the feed antenna are joint simulation. The results show that the antenna system has a focal spot of 50 mm at a distance of 2.81 m from the lens and a focal spot of less than 60 mm at ±5° off axis. The designed antenna system has good focusing performance and a large offset working angle, which can be applied to passive millimeter wave near-field imaging applications.