The invention relates to a preparation method of polyethylene tetrazole and belongs to the technical field of polymer synthesis. The method comprises steps as follows: polyacrylonitrile, ammonium chloride and sodium azide are added to a mixed solution of water and DMF (dimethyl formamide) sequentially, and then, the mixture is heated to 100-150 DEG C and subjected to a reflux reaction for 4-7 hours under the stirring condition; a diluted hydrochloric acid solution is added to the reacted mixed solution and stirred until white precipitates are completely separated out, the precipitates separated out are washed with water and dried, and polyethylene tetrazole is obtained. According to the method, the phenomenon that a middle product has jelly precipitation in the preparation process is avoided, uniform stirring is facilitated, and mass transferring and heat transferring in the reaction process are promoted; the method adopts a simple process, the reaction conditions are mild, and the product yield is high.