The invention relates to a method and a device for dissolving fly ash slurry out by a hybrid acid heating method. The method comprises the following steps of: preheating the slurry, namely preheating proportioned fly ash and hydrochloric acid mixture slurry in a preheating device, and raising the temperature to be between 40 and 70 DEG C; heating the slurry, namely heating the preheated slurry in a reaction kettle by using live steam until the temperature is between 140 and 160 DEG C, and preserving the heat for 0.5 to 2 hours; and performing concentration temperature reduction on the reacted slurry dissolved out from the reaction kettle in a flash evaporator until the temperature is between 118 and 125 DEG C, and feeding the cooled slurry into a diluting tank. The method and the device have a core status in the whole process for preparing aluminum oxide by a fly ash acid method. By the process, the aluminum oxide in the fly ash is reacted with hydrochloric acid and is dissolved out in an aluminum chloride form, aluminum chloride is added into solution, and the aluminum chloride solution provides basic raw materials for subsequently preparing the aluminum oxide.