Disclosed is a method of high purity ammonium dimolybdate produced by the pressure oxidation of low grade molybdenite concentrates. The method includes (a) forming the low grade molybdenite concentrates into the aqueous serosity; (b) oxidating the serosity in the ambience containing the free phenol under the pressure 50 p.s.i. and the temperature under 200 DEG C to generate the first drainage; (c) leaching the first drainage using the aqueous alkali to generate the second drainage; (d) separating and filtering the second drainage to generate the filtrate containing the soluble molybdenum and a solid filter cake containing the unsoluable molybdenum; (e) performing the solvent extraction using the organic solvent, reclaiming the molybdenum from the fluid filtrate, leaching the organic phase by the ammonium hydroxide to generate the first mother liquor; (f) crystallizing the first mother liquor containing the leached molybdenum to generate the crystallization and the second mother liquor; (g) reclaiming the high purity ammonium dimolybdate from the crystallization.