The rotating rectifier is a weak link in the brushless excitation generator and is susceptible to diode open-circuit faults. Multi-tube fault will lead to significant fluctuation of generator excitation current and induce power oscillation of the unit. It is of great significance to study the detection method of diode open-circuit fault. The diode conduction law of the rotating rectifier system of the exciter was investigated. Based on the change of the leakage magnetic field of the armature slot in different operating states of the normal and open-circuit faults of the rotating diode, it was proposed to fix the magnetic field probe in the stator of the exciter to capture the leakage flux of the armature slot. After the diode open-circuit fault, the induced voltage of the probe will change significantly. Then a two-dimensional electromagnetic simulation model of a 5.8MW three-phase brushless exciter was established. The finite element simulation shows that the armature slot leakage flux signal contains the diode open-circuit fault characteristics.The on-line acquisition and feature extraction of the slot leakage magnetic field of the discontinuous conduction AC armature winding of the brushless exciter were carried out. It is proved that the armature slot leakage flux signal can be used to detect the open circuit fault of the rotating diode on-line, which provides a new technical means for the fault detection of the rotating rectifier.