The invention discloses a preparation method of ammonium trithiotungstate and belongs to the field of preparation of high-purity chemicals. The production method includes: reacting a tungsten compound with ammonium sulfide solution, discharging ammonia gas produced in the reaction process from a reaction liquid by means of evacuating in the reaction process or by means of blowing an inert gas into the reaction liquid so that reaction equilibrium tends to a product ammonium trithiotungstate, and adding suitable ammonium salt to the reaction liquid to promote dissolution of the product ammonium trithiotungstate so as to prepare the high-purity ammonium trithiotungstate. The method has the advantages that waste emission is little, using irritating ammonia gas and toxic and stinking hydrogen sulfide gas is not required, process conditions are simple, a process is environment-friendly, and yield is up to higher than 98%; high-purity ammonium trithiotungstate products of gram level reaching tonnage can be efficiently produced according to the method, and industrial large-scale production is facilitated.