The invention relates to a method for preparing spherical nano silicon dioxide and belongs to the technical field of nano material preparation. The method comprises the following steps: firstly, by taking eucalyptus as a raw material, crushing into granules, mixing with river bed mud, carrying out slight fermentation under conditions of high temperature and high moisture so as to obtain micro fermentation eucalyptus granules, further carbonizing, carrying out corona treatment so as to obtain a self-made shape controlled mold plate, preparing nano silicon dioxide by using a homogeneous precipitation method, adding a self-made shape control agent in the preparation process, and calcining after preparation is completed, thereby obtaining the spherical nano silicon dioxide. The spherical nanosilicon dioxide prepared by using the method is uniform in particle size and not liable to agglomerate, has a spherical morphology and has wide application prospects.