Traditional spherical and aspherical surfaces offer limited degrees of freedom for optical system design. Freeform surface breaks the geometric constraints of rotational or translational symmetry. It can correct the aberrations in non-rotationally symmetric systems while decreasing the system size, mass, and number of elements in optical design. The system specifications, configurations, and functions which are difficult to be realized by traditional spherical or aspherical systems can be achieved by using freeform surfaces. The use of freeform surfaces not only offers great potential in the development of the optical design field, but also introduces new difficulties and challenges. This review briefly summarizes the current status of research on the freeform imaging system design. The commonly used types of freeform surface mathematical expression and the aberration theory of freeform imaging systems are demonstrated. The design methods of freeform imaging systems and the applications of freeform surface in various imaging systems are summarized. Finally, the future research directions of the freeform imaging system design are discussed and analyzed.