Abstract: A novel method based on segmentation and iteration of sub-surface is presented as the description of optic free-form surface, aiming at the complex and relatively smooth character of its figure. Three steps are executed in the whole process of ray tracing of free-form surface. Firstly the free-form surface is divided into some sub-surfaces by discrete points. Then the target sub-surface is determined based on the angle sum of a triangle theorem. And finally the iterative algorithm is utilized on the target sub-surface. Even asphere, biconic surface, extended polynomial surface and a single lens system with a freeform surface are set into simulation. The comparison between the result of the method presented and the result of ZEMAX shows that the method presented is of high precision, good versatility and high speed.