The invention belongs to the field of comprehensive utilization of solid waste, and discloses a method for preparing hydrophobic silicon dioxide aerogel from fly ash acid process aluminum extraction tailings as raw materials. The method comprises the following steps of performing high-temperature calcination on the fly ash acid process aluminum extraction tailings; performing washing by distilled water; performing strong alkali leaching; then, regulating the pH value of the leaching liquid by inorganic strong acid and performing still standing; performing drying to prepare gel; performing aging, silanization surface modification treatment and washing on the obtained gel; finally, performing normal pressure drying to obtain the hydrophobic silicon dioxide aerogel material with excellent property. The waste is sufficiently utilized; the characteristics of simple process and low cost are realized; the method is applicable to scaled production and application; the prepared aerogel has large specific surface area, high porosity and excellent performance.