A device for converting carbon dioxide in flue gas into natural gas using dump energy. The device includes a transformer and rectifier device, an electrolytic cell, a turbine, a carbon dioxide heater, a primary fixed bed reactor, a secondary fixed bed reactor, a natural gas condenser, and a process water line. An outlet of the transformer and rectifier device is connected to a power interface of the electrolytic cell, a gas-liquid outlet of a cathode of the electrolytic cell is connected to a gas-liquid inlet of a hydrogen separator, and a liquid outlet of the hydrogen separator is connected to a liquid reflux port of the cathode of the electrolytic cell.