High-alumina fly ash (HAFA) is a waste produced by coal-fired power plant.The main phase of HAFA includes mullite, amorphousSiO2 and c-A12O3.Traditionaly, AmorphousSiO2can be partly removedby NaOHinpredesilicationprocess, while thezeolite phaseis formed on the partical surface spontaneously and preventsremovement of amorphousSiO2.Therefore, a new technology was developed to remove zeolite phase and impurities by acid treatment process, and further prepare mullite by deep desilication process.This new technology has been industrialized by a 3000 ton device.