A preparation method includes 1) dispersing cellulose nanocrystal in water and adjusting pH to 7; 2) adding carboxylate to the aqueous dispersion of the step 1), and stirring until uniform; and 3) adding a monomer and a ceric ammonium nitrate initiator to the system of the step 2), reacting for 0.5-3 h to obtain a precipitate, and subjecting the precipitate to suction filtration, washing, and drying to obtain the cellulose nanocrystal powder. By adding a small amount of carboxylate into a cellulose nanocrystal graft polymer modification system initiated by ceric ammonium nitrate, hydrolysis of cerium ions can be inhibited through complexation of the carboxylate to the cerium ions which play an initiating role in ceric ammonium nitrate, so that ceric ammonium nitrate can initiate the polymerization reaction under acid-free conditions, thereby achieving polymerization of polyvinyl acetate monomer on the surface of cellulose nanocrystals.