Abstract: The air gap magnetic field of the permanent magnet synchronous generator is difficult to adjust. In order to solve the problem and maintain its brushless advantages, a brushless hybrid excitation synchronous generator（HESG） based on harmonic excitation was proposed. On the basis of the HESG, this generator had added two sets of additional harmonic windings: stator harmonic excitation winding and rotor harmonic winding. The harmonic magnetic field established by the stator harmonic excitation winding induced the harmonic electromotive force（EMF） in the rotor harmonic winding, and the harmonic EMF was directly provided to the rotor excitation winding through the diode rotating rectifier. The basic structure was expounded, and the generation mechanism of the rotor harmonic EMF and the excitation principle were analyzed. Theoretical calculation and experimental results show that the air gap magnetic field and terminal voltage of the generator can be adjusted by controlling the stator harmonic excitation current. At the same time, the correctness of the proposed brushless harmonic excitation principle and the feasibility of the scheme are verified.