The invention relates to the chemical field and discloses a preparation method of acrylamide modified grafted corn cob. The preparation method comprises the following steps: the agricultural waste corn cob is used as a raw material, maleic anhydride is used to modify the corn cob, then ammonium persulfate is used as an initiator, N,N-methylene diacrylamide is used as a crosslinking agent to perform graft copolymerization with acrylamide to prepare the modified corn cob. The product has good stability and flocculation performance, the flocculating rate is up to 89.66%, and the product can be used as a flocculant; and the product has better development and application prospects in the water treatment field. By adopting the method, agricultural resources can be utilized reasonably, the addedvalue of the agricultural product can be increased, environment protection can be favored and the sustainable development strategy is beneficial to realization.