Optical freeform surface requires submicrometer form accuracy and nanometer surface finish. Ultra-precision raster milling is an emerging technology in the fabrication of those surfaces in which the dynamics factors are vital to achieve the surface quality. This paper presents a theoretical dynamics model for ultra-precision raster milling. The cutting force is derived in the depth of cut (DOC) planes in the feed and raster directions. Hence, a 3D cutting force model is established. The cutting force induced deflection between tool and workpiece is determined which can be employed to analyze the influence of the deflection on the surface generation in raster milling. The dynamic model is useful for modeling of surface generation and further control of vibration between the tool and the workpiece.