Based on the study of the characteristics of the asynchronous operating power system, this paper provides a new method of out-of-step protection. This new method consists of three components: out-of-step element diagnostic detection, out-of-step region judgment element and out-of-step trip timing selection element. The first element(out of step diagnostic element) uses differential angle and slip angle between two ends to determine whether power system begins to oscillate. To increase the reliability, the positive sequence voltage and current are used as an auxiliary condition. The second element uses the principle of regional distance relay to judge the oscillation area. The third element uses Ucos principle to count the number of out-of-step, and use differential angle as an auxiliary criterion to select the best opportunity to trip. When these three elements operate at the same time, the out-of-step protection will operate immediately. This new method is accurate and reliable and is not affected by system operation mode, and has been verified on RTDS simulation system.