Abstract: Freeform surface provides more degrees of freedom for optical system and stronger capability of aberration correction to break the limitation of traditional surface characterization and system structure, which helps engineer to achieve large field of view, large aperture, miniaturization, and lightweight optical system design while improving the imaging quality. A favorable starting-point of freeform surface imaging optical system design can release the potential of freeform surface to correct aberration and improve the efficiency of system design. Comparing to coaxial optical systems, few examples can be referred to for freeform surface imaging optical system and theory for freeform is not perfect yet. Therefore, how to construct and solve a favorable starting-point of freeform surface imaging optical system is one of the frontier issues in the field of optical design. Based on the research experience of author team for many years, this review summarizes the current design methods of freeform surface imaging optical system design, and divides them into disturbing coaxial system method, directly solving method, and expending field or aperture method and stitching and fusion method, according to the construction principle of freeform surface. Finally, the problems to be solved in the design starting-point for freeform surface imaging optical system are analyzed and summarized.