The invention discloses a method for preparing ammonium trithiomolybdate, and belongs to the field of high-purity chemical preparation. The method comprises the following step: enabling a molybdenum compound and an ammonium sulfide solution to react, wherein in the reaction process, vacuuming is performed or an inert gas is blown into a reaction liquid so as to discharge out an ammonia gas generated in the reaction process from the reaction liquid, thereby promoting reaction balance to move towards the direction of a product ammonium trithiomolybdate, a proper amount of ammonium salt is added into the reaction liquid to promote separation of the product ammonium trithiomolybdate, thereby obtaining the high-purity ammonium trithiomolybdate with a high yield. The method disclosed by the invention has the advantages that a small amount of wastes can be discharged, irritant ammonia water or toxic and rotten hydrogen sulfide is not used, the process conditions are simple and environmentally friendly, the yield is greater than or equal to 98%, and the like; and by using the method disclosed by the invention, a high-purity ammonium trithiomolybdate product from a gram grade to a ton grade can be efficiently produced, and industrial large-scale production can be easily achieved.