Abstract: Disclosed is a method for preparing iron phosphate and by-product fertilizer using ammonium phosphate. The granulation device for by-product fertilizer production comprises a housing having accommodation space which is divided from top to bottom into a granulation chamber, a material-screening chamber and a temporary material-storing chamber; a blowing component provided between the granulation chamber and the material-screening chamber; and a vibrating screen which is able to vertically reciprocated provided between the material-screening chamber and the temporary material-storing chamber which is connected to a discharge pipe, wherein the upper end of the housing is provided with a detachable cover having multiple spraying devices for spraying the molten material into the housing and connected to an exhaust pipe.