Abstract: This paper presents an study of a three-phase LLC resonant converter. By establishing a fundamental harmonic equivalent circuit model, the voltage gain characteristics and operational modes of the converter are analyzed, and its current-sharing principle is elucidated. The limitations of insufficient boost capability under heavy-load conditions with PFM control are identified. To address this issue, a sub-resonant non-frequency-modulation boost strategy integrated with secondary-side synchronous rectification is proposed. This strategy effectively enhances the voltage gain without altering the switching frequency by advancing the turn-on of the secondary-side rectifier during the discontinuous current phase. Finally, the feasibility and effectiveness of the proposed boost strategy are verified through simulations and practical prototype testing.