[Objective] The electromagnetic torque of multi-phase self-excited synchronous motor (SESM) is generated by the interaction of the excitation magnetic field and the stator winding magnetic field, with the excitation magnetic field established by the harmonic-excitation windings. However, existing studies have not thoroughly discussed the relationship between the harmonic-excitation windings distribution structure and the excitation winding flux linkage, which limits further improvement of the electromagnetic torque. Therefore, this paper focuses on optimizing the harmonic-excitation windings distribution structure to enhance the electromagnetic torque. [Methods] Firstly, by analyzing the excitation principle of the multi-phase SESM, the influence mechanism of harmonic-excitation windings distribution structure on excitation magnetic flux linkage was studied. Secondly, based on the influence mechanism, an optimization strategy was proposed whereby harmonic winding with identical induced electromotive force phase were connected in series, and multiple series branches were then connected in parallel. Thirdly, the coupling mechanism of the proposed optimization strategy was analyzed, enabling decoupling between the harmonic-excitation windings and the stator winding. Finally, an finite element simulation model of the motor was built based on Ansys Maxwell to compare the traditional scheme and optimized scheme. [Results] The simulation results showed that, at the speed of 100 r/min, the optimized scheme achieved a 74.5% increase in the excitation winding induced electromotive force, a 57.2% increase in excitation current and a 56.9% increase in electromagnetic torque compared to the traditional scheme. [Conclusion] The optimized scheme of harmonic-excitation windings structure designed in this paper effectively improves the electromagnetic torque at zero-low speed domain and enhances the excitation effect.