There are many special cutting disciplines needed to research in precision hard cutting process. The plastic side flow on machining surface influences machining surface roughness great. The mathematical model of hump height for surface plastic side flow is built based on the model of precision hard cutting and forming mechanism of surface plastic side flow is analyzed. Effect of cutting feed on the maximum scallop height of machining surface is researched and microscopic observation of surface topography is made through the hard cutting experiment. In certain conditions, the machining surface roughness and the cutting off trace increase with cutting feed. Because of the metal softening, some metal which formed side flow fall off immediately but make plastic flow on the strip edge of machining surface when it flows out tool surface. This research supplied theoretical basis for prediction of hard cutting process surface quality.