<p>During the milling process, chatter greatly reduces the machining accuracy and efficiency of parts and decreases tool life. In previous studies, the stiffness variation (SV) method has been shown to be effective in suppressing chatter. However, most of the studies on SV for milling did not consider the effect of the modulation phase, and the stiffness excitation was applied in both the <i>x</i> and <i>y</i> directions. Meanwhile, these studies are based on the conventional regenerative effect model, which does not consider the effect of process damping. In order to suppress chatter more accurately and effectively, this paper establishes a milling process model that considers both stiffness variation and process damping, and analyzes the effect of stiffness variation and process damping on milling stability. On this basis, the differential phase stiffness variation (DPSV) method considering phase variation and the unilateral stiffness variation (USV) method considering stiffness modulation in a single direction are proposed. Simulation results show that the enhancement of stability by DPSV and USV is overall better than that of SV. In addition, the effect of different modulation parameters of DPSV and USV on milling stability is analyzed separately. The effectiveness of the proposed chatter suppression methods is verified by time-domain simulation.</p>