The invention discloses an out-of-step separation criterion based on an actually measured disturbed trajectory changing trend. According to the criterion, based on a phasor measurement unit PMU, mapping a multicomputer system actually measured disturbed trajectory on a one-dimensional expansion phase plane through dimension reducing mapping; an erroneous judgment problem caused by a rough first order derivative is overcome; a dv/dR-R disturbed trajectory is subjected to real-time fitting by adopting a least square method principle; according to an angular velocity-angular radius v-R disturbed trajectory and a dv/dR-R disturbed trajectory changing trend that the first order derivative is changed along R, an out-of-step condition of a system is judged in real time and evidences are provided for starting out-of-step separation control measures. The criterion does not need to be grouped and erroneous judgment caused by individual discrete point errors can be avoided; and the criterion has accuracy and rapidness.