The invention discloses a preparation method of SiO2 aerogel. The preparation method comprises the following steps: fly ash acid sludge is subjected to alkali dissolution, and a sodium silicate solution is obtained; SiO2 hydrosol is prepared through resin exchange and addition of a template agent; the hydrosol is left to stand and aged, hydrogel is formed and soaked in n-hexane for solvent exchange, and the template agent is removed after solvent exchange; finally, the hydrosol surface is modified with a silane coupling agent and dried, and the SiO2 aerogel can be obtained. According to the method, solid waste, namely, the fly ash acid sludge is used for replacing an expensive organosilicon compound to serve as the raw material, the template agent is added, the ordered mesoporous SiO2 aerogel can be prepared with an ambient pressure drying or vacuum drying method and a surface modification method, the production cost and equipment cost are reduced greatly, the safety of a drying process is improved, and the product performance is excellent.