The invention relates to a preparation method of adiponitrile. The preparation method comprises the following steps: firstly performing a reaction on adipic acid and ammonia at 155-200 DEG C to produce diammonium adipate, and then dehydrating under a catalytic reaction of phosphoric acid or ammonium phosphate to produce adiponitrile; a product can be further separated in a separation tower. The reaction is performed on the adipic acid and the ammonia gas at the low temperature of 155-200 DEG C to produce H4NOOC(CH2)4COONH4 first, and then high-temperature cyanation is performed, so that the opportunity of producing a by-product and a coke from the adipic acid at high temperature is effectively avoided, not only the yield is increased by 3-5%, but also the operation cycle of a production device is prolonged by 30-60 days, and the production per unit time is increased. Liquid phosphoric acid is used as a catalyst, firstly ammonium salt of the phosphoric acid is produced, and the ammoniumphosphate can be mixed with ammonium salt of the adipic acid very well, so that the catalytic performance is improved better and the selectivity of the adiponitrile is increased by more than 2%.