In metal machining, regenerative chatter seriously affects tool life, part surface quality and cutting efficiency. To suppress chatter generated during milling, researchers have proposed a variable spindle speed technique. But the previous VSS milling model is based on two degrees of freedom system (two-DOF). In this paper, a three-DOF system suitable for VSS milling of thin-walled parts is proposed by introducing the axial contact angle. The improved semi-discretization method is used to solve the dynamical equation and obtain the stability lobe diagram (SLD). Comparing the SLDs under variable speed and constant speed, it shows that the VSS can obtain more stable regions than that of constant spindle speed (CSS). The experimental results show that VSS milling can suppress chatter.