This invention relates to a method for preparing acrylonitrile by ammoxydation using a multi-story quenching tower. The method comprises the steps of supplying the reacted gas into the first quenching chamber of the multi-story quenching tower, said first quenching chamber has a water supply pipe equipped with spray nozzles at a two-dimensional density of more than 2 nozzles/m.sup.2 for the sectional area of the multi-story quenching tower and contacting the reacted gas with 5 T or more of water per 1 T of the reacted gas fed from the nozzles.