Abstract: The variable magnetic saliency ratio of main machine (MM) in aircraft brushless synchronous starter/generator (ABSSG) makes it difficult to obtain the accurate initial rotor position without a position sensor. In order to improve the estimation accuracy, this paper proposes a novel initial rotor position estimation method for the ABSSG based on the multi-stage-structure characteristics. It uses the fact that the mutual inductance between the stator and rotor windings of MM changes with the change of rotor position. As there is no brush and slip ring in the ABSSG, voltage signals are imposed in the stator windings of the MM, and the response current signals extracted from the stator windings of the main exciter (ME) are used to estimate the rotor position. The proposed method included two steps: first, high frequency (HF) voltages are injected to estimate the initial rotor position without the sector information; after that, low frequency voltage is used to identify the sector. Optimum frequency of the injected HF voltage is obtained through the Fourier analysis of ME stator currents. Finally, experimental results verify the feasibility and effectiveness of this method.