The influence of delay time on milling system stability is analyzed and the root cause of the occurrence of regenerative chatter is revealed. Regenerative chatter results from the delay time feedback of the internal system. The Routh criterion is applied to analyze the variation of damping and stiffness of system before and after the occurrence of chatter, and the limitation depth is obtained. The experimental results indicate that this analytical method is simple and feasible, and the physical nature of chatter can be clearly stated. Lastly, the method′s practical value for engineering can be demonstrated according to relevant cases.