Abstract: During the power system swing, the measured impedance of the transmission line distance protection may enter the protection action area, resulting in the wrong action of the distance protection. At present, the power system swing blocking is adopted in the line distance protection Ⅰ and Ⅱ sections in China to prevent the wrong action of the protection. However, the setting value of the static stable current needs to be manually calculated, which is difficult to set. In addition, when the line is under heavy loads, it will mistakenly lock the distance protection. Therefor, we analyzed the characteristics of the voltage cosine component (Ucosφ) at the protection installation during system oscillation. The variation range of Ucosφ during system oscillation is −1 p.u., 1 p.u.. In normal operations, |Ucosφ| varies in 0.707 p.u., 1 p.u.. After a line fault occurs, Ucosφ is fixed and the value is small. According to the above characteristic differences, a method is proposed to identify system oscillation and line fault by using Ucosφ. The method uses Ucosφ instead of static stable current, without manual tuning, and adaptively identifies system oscillations and line faults. When the system oscillates and the distance protection may misoperate, the distance protection can be blocked; meanwhile, when the line fails, the distance protection can reliably be in non-block state. The real-time digital simulation system is used for simulation verification.