In the field of space optics which is closely related to aeronautics and astronautics, off-axis reflective imaging systems are attracting more and more researchers’ attention due to their various advantages. Compared with traditional spheres and aspheres, freeform surfaces have higher degrees of design freedom, and are capable of correct a variety of asymmetric aberrations. Therefore, freeform surfaces have full capability of designing the off-axis reflective imaging systems, and furthermore they have the potential to meet some high-end or special requirements in space optics. Improving optical performance and realizing special structure are two main application directions of freeform surfaces in space optics. In this paper, following the two directions and aiming at some real requirements of space optical imaging systems, including low F-number, wide field-of-view, compact structure, having a real exit pupil and afocal system, the authors have selected several representative freeform off-axis reflective imaging systems, and emphatically introduced their system configurations, optical performance, and application advantages in space optics.%在空间光学领域，离轴反射式成像系统凭借其多方面的应用优势，正愈发引起研究人员的关注与开发。相对于传统的球面和非球面，自由曲面拥有更高的设计自由度，能够校正各类非对称像差。随着计算技术的发展，充分具备了设计离轴反射式成像系统的能力，并有潜力满足空间光学领域中的各类高端或特殊需求。文章紧扣自由曲面在空间光学领域提高系统性能、实现特殊结构的两大应用方向，瞄准空间光学成像系统的若干现实需求，包括小F数、大视场角、结构紧凑、带有实出瞳、无焦系统等，从目前相关研究成果中，重点介绍几款具有代表性的自由曲面离轴反射式成像系统，并阐述它们的系统构成、性能指标以及在空间光学中的应用优势，同时提出了应用建议及面临的困难。
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