The invention provides a fly ash comprehensive treatment and utilization method which comprises the following steps: grinding fly ash, and carrying out magnetic separation by a magnetic separator to remove iron; treating the ground material with a hot acid solution, washing, and dewatering for later use; adding a fluorine-containing acid solution into a reaction kettle while stirring, adding hydrochloric acid and/or nitric acid into the reaction kettle, and reacting to generate silicon tetrafluoride gas, wherein the silicon tetrafluoride gas and volatile acidic gas in the mixed acid solution are taken into a cooling kettle along with vapor by negative pressure; carrying out reaction on the gas mixture in the cooling kettle to generate SiO2.nH2O, collecting the SiO2.nH2O in the cooling kettle, removing the residual acidic gas, and drying to obtain the gas-phase silicon dioxide product. The invention also provides a device used in the method. The fly ash and other wastes used as the raw materials are made into the high-added-value product gas-phase silicon dioxide, thereby achieving the effect of changing wastes into valuable substances.