The present invention relates to a method for preparing nanocellulose, wherein the method comprises : (1) adding a cellulose raw material to a molten solution of an anhydride compound, increasing the temperature to the melting point or higher, performing a pretreatment reaction, and filtering same after the reaction is finished to obtain a pretreated cellulose raw material; (2) dispersing the pretreated cellulose raw material in water to obtain a dispersion; and (3) mechanically pulverizing the dispersion to obtain a nanocellulose dispersion. In the present invention, by contacting the cellulose raw material with the molten solution of the anhydride, anhydride molecules enter between elementary unit fibers, and the nanocellulose is obtained by peeling same off under the action of a mechanical force, and no other organic solvent, except the anhydride, is added during the treatment process, the amount of environmental pollution is small, the process method is simple, the energy consumption is low, and the cost is low. In the present invention, the nanocellulose prepared by treating the cellulose raw material with an anhydride is rich in carboxyl groups on the surface thereof, and same also has a good water dispersibility and storage stability.