The invention discloses an energy-feedback LLC resonant converter light load voltage modulation system and a modulation strategy. The system comprises a power supply, a half-bridge converter circuit,a resonant network circuit, a high-frequency transformer, a current doubler rectifier circuit and an output load. In the strategy, the structure of a secondary-side synchronous rectifier tube of the high-frequency transformer is used, when the system works in light load, the phase-shift angle between a primary-side power transistor and a secondary-side transistor of the high-frequency transformeris adjusted, output capacitance feeds energy back to the power supply side, reduction of light load output voltage is achieved; and besides, according to an input and output voltage relation of the system, the output voltage is stabilized at rated voltage. Through the modulation strategy, the problem of output voltage pumping under light load of the LLC resonant converter can be solved, the outputvoltage is stabilized, a hardware cost does not need to be increased, problems that the efficiency is reduced, the application range is limited, output voltage ripples are large and the like do not exist, and the prospect in industrial application is great.