The off-axis reflective optical system overcomes the material limitation of traditional refractive system and has the advantages of large aperture, small volume, lightweight and non-chromatic aberration. With the extensive application of freeform surface, which has very high degree of freedom, it has corrected the asymmetric aberration introduced by off-axis system, which has become a research hotspot. Aiming at the complex problem of solving the off-axis three-mirror initial structure, this paper introduces several common design methods of off-axis reflection system, which can be summarized as two ideas from the design principle: first, the initial structure of coaxial system is established according to aberration theory, and the off-axis optimization of optical system is carried out to achieve non-central occlusion; the second is to directly construct a simple optical system without central obscuration, using the complex surface for aberration optimization in the optimization process. According to the two kinds of design theory, the optical systems with different structures are designed as examples. The optimization process and image quality evaluation function show that both methods can meet the design targets, but the optimization process is different, and the construction results are almost equivalent. Among them, the first method is more advantageous in aberration optimization, which is suitable for optical systems with small off-axis angle; the second method avoids complex off-axis optimization and is suitable for compact optical systems.