Abstract: With the flexible spatial layout and the degree of design freedom of the free form surface (FFS), the structure of the optical system was simplified and the quality of the image was improved. The method of using user defined surface (UDS) function in CODE V and computer dynamic link library (DLL) technique to design an optical system incorporating with FFS is introduced through a design example. The accuracy and reliability of the approach are verified by the design example of an optical system with spherical and conic surfaces designed by CODE V. An optical system with a FFS prism is also designed with the method, and it made the optical system light in weight and compact in size. The approach expands the functions of CODE V and effectively enlarges its capacity to calculate complicated surface.