Accurate information about the field current of the wound-rotor synchronous starter generator (WRSSG) is prerequisite for optimal start control research for WRSSG in the starting mode. However, as the field winding of WRSSG is in the rotating parts and there is no slip ring and brush mounted in the shaft, the measurement of the field current is not possible. Afield current estimation method for the WRSSG from the brushless exciter side was proposed in this paper. In this estimation method, the rotor currents and voltages of the asynchronous brushless exciter were first estimated and then used to calculate the field current of WRSSG through the parametric average-value model of the rotating rectifier. Experimental results verified that the proposed estimation method showed sufficiently accurate estimation performance both in steady and transient states.