Abstract: Traditional out-of-step separation criterion based on the phase angle principle is used to distinguish short-circuit fault from the out-of-step oscillation,through tracing the changing trajectory of phase angle between current and voltage which are measured at the installation location of the equipment.Due to the large time constant of 1 000 kV ultra-high voltage（UHV） AC lines,the out-of-step separation equipment based on the phase angle principle may mal-operate in some cases of three-phase short-circuit faults.In view of the above phenomenon,an improved out-of-step separation strategy obtained by the in-depth analysis of the current and voltage characteristics between short-circuit fault and out-of-step oscillation in UHV transmission lines,is proposed.Finally,a real-time digital simulation（RTDS） model based on typical wiring scheme for 1 000 kV long and short distance lines recommended in State Grid Corporation enterprise standard,is set up,and the effectiveness of the improved solution is verified by the test with RTDS model.