Chatter is a strong self-excited vibration phenomenon which is produced by the interaction between the tool and workpiece in the metal cutting process. It can reduce the surface quality of the workpiece and tool life when increase the cutting force amplitude increase and cause strong fluctuation. First, the dynamic milling force system is designed based on the stability prediction analysis. The chatter stability in the milling process is simulated after the study of multi-hardness mosaic hardened steel for its lobes figure. Then, the influence of modal parameters on the shape of lobes figure is analyzed. The forecast accuracy of stability lobe figure is validated by the study of time domain analysis, tool wear and surface topography. The result provides reasonable processing parameters for milling multi-hardness mosaic hardened steel and theoretical basis and technical guidance for the selection of processing parameters in milling automobile cover mold.