Copolymer Starch-PVA coating material can produce a lot of foam on the process of coating fertilizer.Foam can make a small hole on the surface of fertilizer,So the fertilizer slowly release function lowered.We expatiate antifoaming agent's defoaming mechanisms by studying the defoaming effects of the different category,and makes sure the ciganosilance type and polymer-silicon dioxide type antifoaming agent be the best,reducing membrane's absorbing water capacity and penetrating water capacity,thus slow-release effect of coated fertilizers was raised.