The invention discloses a three-stage synchronous motor rotor position estimation method based on a direct decoupling mode. A main exciter of a three-stage synchronous motor uses single-phase alternating current excitation with constant frequency, and a second harmonic wave generated by a rotating rectifier as a high-frequency signal is indirectly injected into an excitation winding of a main generator, a second-order generalized integrator is used for extracting a high-frequency response signal and an orthogonal signal thereof from current loop output controlled by starting of the main generator, high-frequency sine and cosine signals extracted from a stator side of a main exciter are combined, a decoupling signal synchronous with the high-frequency response signal is generated through analytical operation, a heterodyne method is combined for modulation, and a rotor position angle of the three-stage synchronous motor is estimated. According to the rotor position estimation method provided by the invention, the decoupling signal synchronous with the high-frequency response signal can be accurately generated, the calculation amount is small, implementation is easy, the complexity of a motor control algorithm can be effectively reduced, and the accuracy of the obtained position estimation angle is relatively high.