Abstract: Fast out-of-step separation criterion applied in single measurement side is proposed to reduce the application amount of the device and computation amount based on current fast out-of-step separation criterion. The changing characteristics of correlative variables of fast out-of-step separation criterion on leading end and laging end are studied based on equivalent two-machine out-of-step oscillation system.The power angle and active power criterion for out-of-step modeare improved to solve the problem that the out-of-step oscillation on measuring nodes at laging end of line cannot be identified correctly by fast out-of-step criteria. An actual power grid model is simulated by using BPA software, and simulation results prove that the line out-of-step can be distinguished by improved fast out-of-step separation criterion in each end of line.