The multiphase self-excited synchronous motor achieves brushless excitation in the zero-low speed domain by injecting high-frequency harmonic current excitation into the stator side. However, existing research has not fully explored the phase of the injected high-frequency current, limiting further improvement in excitation current and torque at zero-low speed domain. To address the above problems, this paper first derived mathematical formulas based on the principle of fractional slot concentrated winding magnetomotive force, and analyzed the mechanism of high-frequency current injection for excitation. Next, the effect of the high-frequency current phase on excitation performance was investigated, and a phase optimization strategy was proposed. Finally, a self-excited motor model was established in Maxwell, and finite element analysis was conducted to verify the proposed strategy. The research results showed that with the high-frequency current phase optimization strategy, at 100 r/min, the induced potential increased by 17.5%, the excitation current rose by 7.5%, torque increased by 6.6% and the torque ripple decreased by 71.58%.