The invention relates to a preparation method of dodecanedioic nitrile. According to the method, dodecanedioic acid is added into a nitrilation reactor; ethyl methylimidazole dihydric phosphate ionic liquid is added, and the dosage of the ionic liquid is 5 to 50 percent of the volume of the dodecanedioic acid; heating is performed to 140 to 190 DEG C; after the dodecanedioic acid is completely melted; ammonia is introduced in a stepped way; then, temperature rise is performed for reaction; after the reaction is completed, dodecyl dinitrile crude products are subjected to distillation purification; the dodecyl dinitrile is obtained. The method has the advantages that the reaction time and the reaction temperature are lowered to a certain degree; the side reaction of the reaction is reduced; the yield is improved.