An excitation system with a multiphase angular brushless exciter is the preferred nuclear power excitation scheme because of its low output voltage ripple. However, interturn short circuits of the stator field windings (ISCFW) often occur in the brushless exciter, affecting the normal operation of the system. To study this type of fault, this paper presents research on theoretical analysis, simulation calculation, and model machine experiments. First, through the analysis of the magnetomotive force produced by windings and their interaction in the air gap under fault, the frequency domain characteristics of the postfault steady-state current are obtained. A general multiloop-method-based mathematical model of multiphase angular brushless excitation system with ISCFW is established. Correlated simulations and experiments with ISCFW are carried out, verifying the result of the theoretical analysis: mP times harmonics are in the field current, while all harmonics are in the armature current. The general theoretical analysis method and mathematical modeling method proposed in this paper provide a basis for the protection scheme to rapidly remove ISCFW that occurs in the multiphase angular brushless exciter.