Chatter is known as a main factor that limits the machining quality and efficiency, and one universal solution is to predict occurrences of chatter via calculating the stability lobe diagram (SLD), such as time-domain methods, which are relatively time-consuming. Thus, based on time-domain methods, a boundary auto-location algorithm for the prediction of SLD in milling is proposed. In the proposed method, by setting a series of judgements based on the state of the transition matrix of the dynamic system, the calculation trajectory automatically surrounds the stability boundary line except isolated islands. Only the points on and around the stability boundary were calculated to draw SLD. The contrast simulations were conducted to verify the calculation efficiency of the given algorithm. And the results show that the computational time of the proposed method was cut down significantly than that of the traditional method.