The invention provides a preparation method of silicon dioxide ultrafine powder, comprising the following steps: controlling dried chemical additives by oxalic acid to increase the strength of gelskeleton, improving pore canal structure to lead the pore canal to be homogenized and better resist the action of capillary tension, using surface active agent to lead the colloidal particle to be in microemulsion and controlling the growth of the colloidal particle. For the obtained silicon dioxide nano ultrafine powder, the stacking density is 0.17-0.23g/cm<3>, the specific area is 342-389m<2>/g and the porosity is 85-94.5%. In the invention, expensive equipment and complex technique required in supercritical processing is avoided, various conditions and technical parameters are easy to control, the preparation cycle of powder is short nearly without byproducts, large quantity of replacement solution and defects in blocks prepared in constant pressure. The method uses cheap water glass as silicon source, the price of which accounts for one tenth to one twentieth of that of tetraethoxysilane or tetramethoxysilane, and the toxicity is far smaller than that of tetraethoxysilane and tetramethoxysilane.