This paper designs an LLC converter with super-wide output voltage. A variable-mode control strategy with three operating modes is adopted for the converter, and a novel optimal parameters design method based on this strategy is proposed. The designed converter can achieve a super-wide output voltage range at various load without increasing circuit's complexity, expanding the operating frequency and phase-shifted range. The magnetizing inductance is maximized to decrease magnetizing and conduction losses on condition that the requirements of wide gain and primary MOSFETs' soft-switching are met, so the converter can maintain a high efficiency within the entire output voltage range. Finally, a 2.5 kW experimental prototype with output voltage ranging from 25 V to 150 V is set up, by which the effectiveness of the strategy and the design method are proved.