Ultrasonic-magnetorheological combined (UMC) finishing is a new technique for the ultra precision machining of aspheric surfaces, especially for high quality workpieces with small-radius concave surfaces or freeform surfaces. The characteristic of UMC finishing is discussed, and an experimental equipment for UMC finishing with 5 axes is developed. According to requirements of deterministic removal in UMC finishing and the experimental equipment, post processing algorithm for aspheric surfaces is researched. The derivation process and computer simulation result of the algorithm is presented. The error analysis is conducted and the error control method is also proposed. A series of experiments have been conducted, and experimental results show that high quality surfaces can be achieved by the post processing algorithm.