Recently, the high quality and high productivity in fabrication of freeform optics has been of primary interest in manufacturing industries, such as die and mould manufacturing, aerospace part manufacturing, and so forth. However, the fabrication of freeform optics is currently expensive and vastly complex. Ultra-precision raster milling can produce non-rotational symmetric surfaces with sub-micrometric form accuracy and nanometric surface finish without the need for any subsequent post polishing. While, there is little research work focus on this kind of machining method. This paper presents a framework of a tool path generation system for freeform surface ultra-precision raster milling. This system includes model of freeform optics, tool path generator, interference monitor and an optimization model of machining parameters. The tool path generation system can generate interference free and optimal tool path for machining freeform surfaces. Some simulation results have been presented to illustrate the performance of the system.