The invention discloses a preparation method of spherical silicon dioxide, and belongs to the technical field of chemical synthesis. The method specifically comprises the following steps: introducingsodium silicate and carbon dioxide-containing gas into a micro-reactor, carrying out carbonization reaction, aging and separating to obtain a precipitate, and conducting washing and spray drying to obtain spherical silicon dioxide. The spherical silicon dioxide prepared by the preparation method is high in specific surface area, large in pore volume and good in dispersity, and has a uniform mesoporous structure; and the preparation method has the advantages that the raw materials are easy to obtain, the production cost is low, the process is stable and controllable, and continuous production is easy to realize, and has remarkable advantages in large-scale production of high-quality silicon dioxide.