The successful development of the surface topomorphy model and the model-based simulation system is helpful in identifying the optimal cutting conditions of machining optical freeform surfaces without the costly trial-and-error cutting tests. This article presents a novel model that describes the path-interval and feed-interval scallops generating mechanism in ultraprecision milling process. The topomorphy value of any point on the machined surface can be calculated directly by simulating the tool kinematics precisely and discretizing the cutting edge and meshing the workpiece. Through analyzing the effect of the feed per tooth on the surface scallop height and simulating the machined surface topomorphy, it can be found that the scallop height increases greatly when the feed/pick ratio is above 1 and the feed-interval scallop plays a more important role to surface roughness than the path-interval scallop in ultraprecision milling process.