Arotating diode failure is themost common electrical fault in brushless exciters. It brings a safety hazard to the generator excitation system so that the whole unit is forced to be shut down. It is necessary to detect and deal with this fault accurately. According to the electrical structure characteristics of brushless exciters, this paper analyzes the influence of an open circuit rotating diode on the armature current and armature magnetic field. It is proposed here that we install the magnetic field detection coil on the stator yoke and use the rotational frequency component in the induced voltage to diagnose the open circuit fault in the rotating diode online. In this paper, a two-dimensional model of a 22-pole 39-phase brushless exciter is established, and the new detection method is verified using a finite element simulation. This provides a new solution for rotating diode fault diagnosis in brushless exciters. The detection method exhibits good performance in terms of practicability, real-time usage, and versatility.