The invention relates to a high-pressure preparation method of pure-phase molybdenum dinitride. The method is characterized by successively comprising the following steps: (1) taking sodium molybdate,boron nitride or silicon nitride as raw materials, weighing and grinding in proportion, mixing uniformly and then pressing to form a molded product; (2) putting the molded product into a high-pressure assembly, wherein the high-pressure assembly is arranged in a high-pressure press machine for pressurizing and heating, keeping pressure at 1-5GPa, keeping temperature at 400-600 DEG C and keeping heat preservation time for 15-30min, after completing temperature preservation, cooling, relieving pressure and taking out the molded product; (3) washing the molded product obtained in step (2) for 10-60min by using hot distilled water at 40-80 DEG C after grinding, and then centrifuging a solid matter; and (4) placing the solid matter obtained in step (3) into a drying box at 40-80 DEG C to dry for 2-6h to obtain molybdenum dinitride powder; wherein the sodium molybdate and the boron nitride are ground and mixed in a molar ratio of (1.1 to 3.0) to 2.0, and the sodium molybdate and the siliconnitride are ground and mixed in a molar ratio of (2.0 to 2.5) to 1.0. The high-pressure preparation method has the advantages that the method is simple and easy to implement, can be used for quicklypreparing the pure-phase molybdenum dinitride in batches, and thus has a very good industrial application prospect.