Disclosed by the present invention is an optical coherence tomography (CT)-based starting standby transformer open-phase detection method: using a starting standby transformer in a power plant as a standby power source, the same being in idle working conditions for a long time; once a single phase open-phase failure occurs, voltage phasors and voltage sequence components at each side do not experience significant changes; if the defect cannot be found promptly, a severe threat will be posed to the safe operations of the power plant. The present invention employs optical CT to detect a three phase current at a high voltage side of a starting standby transformer. When the current meets an open-phase criterion, it is determined that an open-phase failure occurs, and an alarm signal is provided after a delay to notify an operator for prompt handling, thus improving the operating reliability of a starting standby transformer system at a power plant.