The invention discloses a design method for judging out-of-step separation based on active/reactive variants in-situ. When the mechanical accelerated power of the system exceeds the electric braking power, if the electric braking power continuously reduces while the phase angle continuously increases, the unit is further accelerated and the out-of-step occurs, thus the time is the critical out-of-step time of the system. The design method of the invention is based on principle of conservation of energy and can achieve the safe and reliable out-of-step separation control of the power system in-situ with fast action, thereby preventing power system stabilization destruction and large area blackout caused by emergent events.