Abstract: During the starting process of an aviation multi-stage starter/generator, accurate rotor position of the main machine(MM) is required, while the position sensors are constrained by volume, weight and etc., and thus the research on rotor position estimation is required. In this paper, based on the structure of multi-stage machine, and by analyzing the characterization of rotor position in the stator current of the main exciter(ME), a rotor position estimation method "based on the fixed-point sampling of the direct-axis current of the two-phase exciter stator" is proposed. The method is implemented by three steps during starting process: first, real-time sampling of exciter's two-phase stator current is carried out, and it is transformed to the synchronous rotation dq coordinate system of the stator magnetic field; Secondly, the direct-axis current is sampled at a fixed-point to extract the sine and cosine signals, where rotor position information is contained, and the rotor position increment of ME is obtained by a phase-locked loop. Finally, combining coaxial installation characteristics of the MM and ME with the initial rotor position of MM, real-time rotor position of MM is obtained to realize sensorless starting in the low speed range. This method does not require high-frequency signal injection and filters, and reliance on MM's saliency is also avoided. At the same time, robustness of the phase-locked loop is strong. Experiments verifies the effectiveness of the proposed method and its high accuracy.