The invention discloses a polyurethane grafted cellulose nanocrystal and a preparation method thereof. The method is mainly characterized in that the surface of cellulose nanocrystal is grafted with synthetic polyurethane to make the cellulose nanocrystal stably dispersed in water or an organic solvents or a polymer matrix, and sulfonate radicals are removed in the polyurethane grafting reaction process, so the maximum thermal degradation temperature of the cellulose nanocrystal is increased by 20-200 DEG C, and the problems of poor dispersibility of the cellulose nanocrystal in the organic solvent and the polymer matrix and poor heat resistance in the processing process are effectively solved. The polyurethane grafted cellulose nanocrystal has a wide application prospect.