Freeform surfaces play an important role in compact and complex optical systems. The design and description methods of freeform surfaces are key to their applications. A freeform surface design method based on grid sag is proposed by this paper, with light rays being taken as the research object and imaging optics being combined with non-imaging optics. Non-imaging theory is used to calculate the coordinates of discrete points on the freeform surface, which are then fitted to form a base surface. The deviation between the discrete points and the base surface is set as the compensation amount. Subsequently, imaging optical theory is employed to optimize both the base surface and the compensation amount simultaneously,until the system design requirements are met. Using this method, an optical shaping system with a freeform surface reflector is designed, achieving illuminance uniformity of over 90% and distortion of less than 2%. This method can be applied to the design of freeform surfaces in non-imaging fields such as tunnel lighting, automotive lighting, and photovoltaic concentrating systems, as well as in imaging systems with large field of view, off-axis, and ultra-short throw projection system.