Abstract: In this paper, a novel path-folded near-field quasi-optical antenna is introduced, which is successfully applied to the bi-static measurement of weak scattering from microwave blackbody. In the bi-static measurement, traditionally small aperture antennas are frequently utilized in the low frequency range, and the target under test shall be small enough for ensuring the far-field condition. Different from the traditional design of small aperture antenna based on far-field condition, the design purpose of quasi-optical antenna in this work is to realize focused illumination with flat phase and amplitude edge attenuation in the target area, for reducing the link attenuation, and measuring the far-field scattering characteristics of the target in the near-field area. The design is based on the quasi optical design method, which consists of the main mirror and the flat mirror to form a compact folded optical path. The design process starts at the equivalent Gaussian beam parameter determination of the feeder horn antenna, then the main mirror is designed based on the Gaussian beam propagation methodology, and further the folded path is examined by full-wave simulations. The simulation results show that good focusing effect can be achieved even if the basic conical horn feed is used. The bi-static measured data show that the design of the antenna achieves the design purpose, and provides a key beam control for the development of the near-field bi-static scattering measurement of sub-millimeter wave, which is a good reference for applications.