Abstract: The concept about the loop-line on an open-loop amplitude-phase characteristic curve is discussed in this paper, by which the necessary condition for judging the closed-loop's unstability is obtained, the corresponding relationship between the reversed loop and the open-loop poles on the right half of the plan is definitely established, and the flexibility of the stability judging method and the rationality of its procedure are increased. Also, it has a certain ability to distinguish between the real and the false of the given condition.