[Objective] To address the issue of high harmonic content in the air-gap magnetic field and deteriorated induced voltage waveform caused by the traditional constant slot-tooth width ratio structure, this study designs an axial-flux counter-rotating dual-rotor synchronous generator (CR-DRSG) with a stator-radially variable slot-tooth width ratio (S-RVSWR). [Methods] A three-phase 6-slot fractional-slot concentrated winding modular stator was proposed, featuring a variable slot-tooth width ratio structure where the tooth width remained constant and the slot width increased linearly with the stator’s circumferential radius. Distributed-winding rotors with 24 slots were placed on both sides of the stator, with dominant pole pairs of 2 and 4, respectively. The magnetic fields generated by DC excitation of the dual rotors were coupled with the stator’s dominant pole pair fields. The integral cancellation mechanism of non-working harmonic fluxes was derived using the magnetomotive force-permeance model and radial integration theory. The electromagnetic performance of the traditional and new structures was compared and analyzed through three-dimensional finite element simulations and a prototype experiment platform. [Results] The simulation and experimental results demonstrated that the S-RVSWR structure significantly improved the induced voltage waveform, making it closer to a sine wave. Compared to the traditional constant slot-tooth width ratio structure, the total harmonic distortion of the no-load phase voltage (5 Hz) was reduced from 24.31% to 5.20%, with the amplitudes of the 5th, 7th, and 11th harmonics notably decreased. [Conclusion] The structure effectively suppresses low-frequency harmonics in the induced voltage by leveraging the integral cancellation effect generated by the stator’s radial geometric non-uniformity. And the effetiveness of the S-RVSWR design has been verified.