The invention discloses a preparation method of ATTM (Ammonium Tetrathiomolybdate), and belongs to the field of preparation of high-purity chemicals. The preparation method is characterized in that a compound of molybdenum is enabled to react with an ammonium sulfide solution, and an ammonia gas generated in a reaction process is discharged from a reaction solution through a way of vacuumizing during the reaction process or blowing an inert gas in the reaction solution, so that reaction equilibrium is facilitated to move towards a product-the ATTM; an appropriate amount of ammonium salt is added in the reaction solution, so that high-purity ATTM is prepared in a high yield by promoting the separation of the product-the ATTM. The preparation method disclosed by the invention has the advantages that the waste emission is less, irritant ammonium hydroxide and a hypertoxic and fetid hydrogen sulfide gas are not needed to be used, the technological conditions are simple and environmenta friendly, the yield is up to more than 98 percent, and the like; according to the preparation method, gram-scale and ton-scale high-purity ATTM products can be efficiently produced, and industrial large-scale production is easily realized.