There are shortcomings of an LLC resonant converter with strong gain load sensitivity and strong coupling with efficiency. Thus this paper proposes a wide-gain and high-efficiency LLC resonant converter topology composed of LLC resonant converter and a two-switch buck-boost converter. Using parallel connection and output series, the LLC resonant converter transmits power and the buck-boost adjusts the output voltage. The LLC resonant converter operates at the resonant frequency, and uses PWM for the buck-boost to regulate the output voltage. The operation mode of the proposed converter is analyzed, the corresponding parameter design method is given, and the simulation verification is carried out. Finally, experiments are carried out on prototypes with input 30 V, output 200~360 V, and 360 W, and the gain range and efficiency of the experimental prototype are 6.67~12 and 97.4%, respectively. Simulation and prototype experiments verify the effectiveness of the proposed wide-gain and high-efficiency LLC converter topology and its modulation method. This work is supported by the National Key Research and Development Program of China (No. 2018YFB1503000 and No. 2018YFB1503001).