The importance of the excitation system as a core component of the synchronous generator set is self-evident, and the current power plant widely adopts high-efficiency rectifier equipment as the core of the excitation system to ensure the stable operation of the generator set. However, in the complex and changing operating environment, the power electronic devices, especially the power diodes, within the 18-pulse autocoupling varactor rectifier, which is particularly critical in the excitation system, are highly susceptible to open-circuit failures. In this paper, we propose an open-circuit fault diagnosis strategy for the power diodes of the 18-pulse-wave autocoupled varactor rectifier based on the average values of phase and cycle currents to diagnose and locate the open-circuit faults of the 18 power diodes in the system by detecting the states of the input branches of each rectifier bridge at a specific phase angle of the supply voltage. Simulations verify that the proposed diagnostic strategy is accurate and robust. The theoretical maximum detection time of this fault diagnosis strategy is about 1 AC power cycle.