A preparation method of a fluorite mineral inhibitor comprises following steps: (1) 30 to 40g sodium amine toluene sulfonate is added to 180 to 200 ml 20% (mass concentration) tannic acid solution; the mixture is heated to 85 to 90 DEG C for dissolving; then the mixture is cooled to 20 to 25 DEG C in ice-bath, and under the temperature, 35 to 40 ml 20% (mass concentration) sodium nitrite solution is added to the mixture to obtain a product 1. (2) a solution is prepared from 15 to 20 g aniline black and 30 to 35 ml 1mol/L sodium hydroxide; the solution and the product 1 are coupled under pH 10 to 10.5 to obtain a product 2; the product 2 and direct black are coupled under pH 10.5 to 11 to obtain powder which is easily soluble in water; the powder is the inhibitor. By means of the inhibitor of the invention, the generation of fluorite mineral can be inhibited while the generation of barite mineral is not inhibited; the inhibitor is good for the floatation separation of fluorite and barite minerals.