The invention discloses silicon dioxide aerogel. According to the silicon dioxide aerogel, sodium silicate is utilized as a silicon source, and the silicon dioxide aerogel is prepared by a two-step hydrothermal synthesis method. The invention further discloses a preparation method of the silicon dioxide aerogel. According to the silicon dioxide aerogel disclosed by the invention, the porosity andthe specific surface area of the silicon dioxide aerogel are effectively improved, time required by gel modifying and solvent replacing for gel shrinkage is reduced, and preparation cost is saved. Compared with other same kind of silicon dioxide aerogel, structural mechanical property of the silicon dioxide aerogel disclosed by the invention is improved, and the silicon dioxide aerogel can be widely applied to the field of thermal-insulation materials. The preparation method disclosed by the invention has the advantages of easiness in operation, moderate preparation conditions and high efficiency; furthermore, the preparation conditions are easy to control.