Although frequency modulation (FM) has been widely used in the LLC resonant converter, the challenges remain in the light-load condition. The nonlinear model makes the control design difficult and the efficiency suffers due to high operating frequency in light-load condition. In this paper, a novel fixed-frequency modulation method is proposed for the LLC resonant converter to address these challenges. The proposed modulation method combines any two of the three chosen operating modes in a control period to regulate the output power. A linear mathematical model can be achieved to ease the control design and high efficiency can be maintained from light to heavy load conditions. Experimental results are presented to verify the performance of the proposed method with a 12V/8A LLC resonant converter prototype, achieving over 20% efficiency improvement for light load condition without any extra or upgraded hardware.