In all kinds of machine tool vibrations, regenerative chatter has the minimum stability critical width of cut. In other words, a machine tool has the worst ability to resist regenerative chatter. Coefficient of Merit (COM) is the crossreceptance value between the cutting force and the relative displacement in the direction normal to cut surface, which can reflect the ability of a machine tool to resist regenerative chatter. Otherwise, in the continuous cutting processes of a machine tool, Coefficient of Merit decreases gradually, and then the ability to resist regenerative chatter is weakened. Based on relative excitation method and fourdegreeoffreedom dynamic model of end milling, effect of the deterioration of the relative dynamic compliance on the change trend of COM and stability Lobe diagram was focused. Research shows that axial critical depth of cut decreases gradually and the trend is accelerated descending, which is consistent with the deterioration trend of COM. Therefore, considering the effect of relative dynamic compliance of a machine tool on stability Lobe diagram, the highefficiency cutting parameters can be chosen only by the premise of guarantee of machining stability.