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.