In face milling process, the cutting chatter induced by unsuitable processing parameters could lead to tremendous damage to machine tools and loss of machined surface quality. Therefore, as one of the most effective and efficient methods to avoid chatter, the stability lobe diagram (SLD) has received continuous concern from researchers. Since the dynamic cutting forces are definitely dependent on the tool path and the stiffness distribution of the workpiece, the exact interaction between the milling cutter and the workpiece should be taken into consideration for chatter prediction. However, most related researches have been focused on the dynamic modeling of the spindles and machine tools, regardless of the characteristics of workpieces. To overcome this limitation, an analytical method is proposed to predict the milling chatter for workpieces with variable stiffness. Firstly, a dynamic model for the general milling process is established. Secondly, the cutting insert engagement is determined based on the stiffness characteristic of the workpieces and the cutting path of the milling cutter. At last, the SLD applicable to workpieces with variable stiffness is constructed, and a case study of an engine block shows that the proposed method is valid.