[Objective] To address the limited excitation efficiency of single-harmonic excitation in multiphase self-excited synchronous machines under medium- and high-speed operating conditions, a multi-harmonic cooperative excitation strategy based on an improved genetic algorithm is proposed. This strategy fully utilizes the high control degrees of freedom of the multiphase machines and increases the rotor excitation current under the condition that the total stator harmonic current is limited. [Methods] First, the harmonic excitation mechanism of an eleven-phase self-excited synchronous machine was analyzed, and a mathematical model describing the relationship between stator harmonic currents and rotor excitation current was established. On this basis, the conventional single-harmonic excitation scheme was extended to a multi-harmonic cooperative excitation strategy, in which the third-, fifth-, seventh-, and ninth-order harmonics were introduced as the main excitation components. Subsequently, with the objective of maximizing the excitation current, a multi-harmonic amplitude allocation optimization model was constructed under the constraint of a constant total harmonic current effective value. An improved genetic algorithm with adaptive mechanisms was adopted to globally optimize the amplitude ratios of different harmonic currents. Finally, the optimal distribution of the fundamental and harmonic currents was achieved by combining with a lookup-table-based method. [Results] The proposed multi-harmonic excitation strategy was verified through finite element simulations and experimental platform. Under identical total harmonic current constraints, the optimized multi-harmonic excitation scheme exhibited superior excitation performance compared with the conventional single-harmonic excitation method. Experimental results demonstrate that the rotor excitation current was increased by approximately 11.5%, which was consistent with the simulation results and confirms the effectiveness of the proposed optimization approach. [Conclusion] The research results indicate that the proposed multi-harmonic cooperative excitation strategy based on an improved genetic algorithm can effectively enhance the excitation capability of the multiphase self-excited synchronous machines without increasing the total harmonic current. This strategy fully utilizes the control degrees of freedom of the multiphase systems, providing an effective solution for efficient excitation control in the medium-to-high speed range.