Preparation method of anastrozole

The invention provides a preparation method of anastrozole. The preparation method comprises the following steps that 1, 5-bromomethyl-a,a,a',a'-tetramethyl-1,3-xylylene cyanide and 4-amino-1,2,4-triazole are taken as raw materials to react in an organic solvent, and crystallization is conducted to obtain an intermediate I, that is, a,a,a',a'-tetramethyl-5-[(4-amino-1,2,4-triazol)-methylene]-1,3-xylylene cyanide bromide; 2, the intermediate I is dissolved into a solvent, a little amount of water, acid and a metal catalyst are added, sodium azide is added in batches in a reflux state, immediate filtration is conducted after a complete reaction, a mother solution is concentrated until the volume of the solution is 0.25-0.75 times of the original volume of the mother solution, cooling crystallization is conducted, solids are filtered and subjected to PH regulation and extraction, and concentration is conducted to obtain a crude anastrozole product; 3, the crude anastrozole product is dissolved into an alcohol solvent, decoloring and cooling crystallization are conducted, and the fine anastrozole product is obtained. According to the preparation method, iso-anastrozole impurities influencing the medicinal efficacy are prevented from being generated from the source, no iso-anastrozole is detected out, the remaining single impurity content is less than 0.05%, and the remaining total impurity content is less than 0.2%.