The excitation system with a multiphase annular brushless exciter (MPABE) is widely used in large-capacity nuclear power plants. However, in field operations, the rotating rectifier of the excitation system often fails due to the damage of diodes, which can cause serious consequences in long-term operations. To better protect the rotating rectifier, an accurate grasp of the characteristics after the rotary rectifier faults becomes essential. Since the rectifier rotates with the rotor at high speed, the rectifier fault cannot be monitored directly. However, it is feasible to monitor the fault according to the characteristics induced in the stator field winding of the MPABE. Hence, this article makes a thorough theoretical analysis of the rectifier fault mechanism to obtain the time-domain and frequency-domain characteristics of the stator field current when the rectifier is under normal operation, one-tube open-circuit fault, one-phase open-circuit fault, and two-phase open-circuit fault. Then, a new rectifier open-circuit fault diagnosis method based on the time–frequency characteristics of the fault excitation current is proposed. The final prototype and field experiments show that this method is simple and reliable, which can be used to monitor and detect the rotary rectifier faults in MPABE systems.