In response to the problem of narrow gain range of traditional LLC resonant converter, this paper proposes a novel LLC resonant converter topology with wide gain range. In this circuit, the output of a four-switch Buck-Boost converter is cascaded with the front bridge arm of the LLC resonant converter. The full-bridge LLC resonant module switching tubes operate near the resonant frequency point, and the input voltage AC amplitude of the LLC resonant module is varied by changing the duty cycle of the four-switch Buck-Boost module switching tubes. The four-switch Buck-Boost module only needs to carry part of the load power to realize the adjustment of the wide voltage gain range of the whole machine. Through the modal analysis and gain derivation of this new converter, the conditions for all switching tubes to realize soft switching and the control method of wide gain are derived. Finally, a 30 V/300 W experimental prototype is constructed, and the peak efficiency of the converter is 95.27%, which is 2.13% higher than that of the traditional two-stage topology. The experimental results prove the feasibility of the proposed topology circuit and the correctness of the theoretical analysis.