The invention discloses a design method for judging out-of-step separation based on application impedance compensation principle. The design method can automatically obtain the real-time impedance value of the line; use the impedance compensation principle to judge out-of-step separation method according to the real-time impedance value as a line compensating impedance value and automatically identify whether the power system on the two sides of the tie line is in an out-of-step oscillation state, the out-of-step center is on the line, and correspondingly open and lock the implement of the out-of-step separation controlled action, thereby achieving the out-of-step separation control which are safe, reliable and fast action and preventing power system stabilization destruction and large area blackout caused by emergent events.