The invention discloses a method for preparing a silica sol, the method comprises the following steps: (1) adding an alkaline catalyst or an alkaline catalyst aqueous solution into a mixture of a silicon source containing elemental silicon and water; and (2) introducing a direct current in the reaction process to generate the silica sol. According to the method, hydroxyl ions are provided by adding the alkaline catalyst, and water ionization is promoted to generate the hydroxyl ions, so that the initial reaction speed of the elemental silicon and the hydroxyl ions is induced, the direct current is introduced in the reaction process of the silicon and water, so that the hydrolysis reaction speed of the silicon is accelerated, and the silica sol is generated. According to the preparation method disclosed by the invention, the hydrolysis reaction speed of the silicon is accelerated by introducing of the direct current, and the mass of the used alkaline catalyst used in the step (1) is very small relative to the mass of the silicon source, so that the content of cations in the finally prepared silica sol is small, and the amount of hydroxyls adsorbed by the cations of the alkaline catalyst on the silica sol shell is reduced.