The interturn short circuit of field windings in synchronous machines will cause the field current increasing, reactive power output decreasing as well as the aggravation of vibration. In this paper, by using the multiloop theory and analyzing the steady-state characteristics of stator and rotor fault currents, the steady-state expression of each loop current is obtained, and then differential equations in steady-state system are reduced into linear equations; according to the principal that components of the same frequency are equal, the steady-state solution of the ac and dc components of each loop current can be obtained and the steady-state calculation of the fault can be achieved rapidly. The field winding interturn short-circuit experiments have been carried out on a 12-kW synchronous machine with three pairs of poles, the results of which agree well with that of steady-state simulations. Based on the aforementioned characteristics analysis and simulation results, an online fault monitoring method in view of the unbalanced current effective value of stator branches is proposed, by which the fault characteristics can be extracted to the highest degree and short circuit with small turns in field winding can be sensitively reflected.