Disclosed is a method for preparing a nanocellulose, the method comprising : (1) dispersing a cellulose raw material in water to obtain a raw material dispersion; (2) placing the raw material dispersion in a sand mill for sand-milling to obtain a sand-milled product; and (3) subjecting the sand-milled product to homogenization in a high-pressure homogenizer to obtain a nanocellulose dispersion. By combining sand-milling and high-pressure homogenization means, the method can obtain a nanocellulose size obtained only by chemical means and on the premise of not needing to add a chemical reagent.