Abstract: To improve the accuracy, stability, and real-time performance of time transfer in power systems, this paper proposes a high-reliability power system timing method based on GNSS (Global Navigation Satellite System). This method supports three timing modes for local rubidium clocks in power systems: remote timing from the atomic time standard maintained by Beijing Institute of Metrology, GNSS system timing, and rubidium atomic clock frequency prediction correction. These modes ensure that power system clocks remain traceable to the national time reference UTC(NIM) during normal operation of time transfer standards or transfer links, during fault conditions, and during special periods when all communication means are interrupted. Comparative data experiments were conducted under all three timing modes. Results demonstrate that the standard deviations and time stability achieved in all three modes meet and exceed the expected synchronization targets with UTC(NIM).