A method for converting carbon dioxide in flue gas into natural gas using dump energy. The method includes: 1) transforming and rectifying a voltage of dump energy generated from a renewable energy plant, introducing the voltage-transformed and rectified dump energy into an electrolyte solution to electrolyze water therein to yield H2 and O2; 2) purifying industrial flue gas to separate CO2 therein and purifying CO2; 3) transporting H2 generated and CO2 to a synthesis equipment, allowing a methanation reaction between H2 and CO2 to happen to yield a high-temperature mixed gas with main ingredients of CH4 and water vapor; 4) employing the high-temperature mixed gas to conduct indirect heat exchange with process water to yield superheated water vapor; 5) delivering the superheated water vapor to a turbine to generate electric energy, and returning the electric energy to step 1); and 6) condensing and drying the mixed gas.