[en] Highlights: • A HAc/NaOH/ultrasonication-assisted joint process was applied to extract SiO₂ from FA. • This joint process was feasible to separate Si/Al effectively. • High purity (98.65%) and high yield (51%) of SiO₂ were obtained. • The extracted SiO₂ exhibited better adsorption prospect to dyes. • This research extended the application and recycling of FA solid waste. Silica extracting from fly ash (FA) becomes a key technology for its comprehensive utilization of waste. Here, high-purity SiO₂ was extracted from FA via organic acid/inorganic alkali/ultrasonication-assisted joint process and characterized by scanning electron microscopy, transmission electron microscopy, Brunauer-Emmet-Teller, X-ray fluorescence spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The optimal extracting conditions were optimized: with acetic acid (HAc) leaching at 25 °C for 1 h and FA/HAc of 1:3; then with 15% NaOH dissolution at 100°C for 2 h and FA/15% NaOH ratio of 1:4.5; cooled and introduced water (5.42 g/100 mL); ultrasonication-assisted for 20 min. The final yield and purity of SiO₂ were 51% and 98.65%, respectively. This joint method could be applied in several various FA raw materials and the extracted SiO₂ exhibited better adsorption prospect to cationic dyes of methylene blue and malachite green, which extended the application and recycling of FA.