The invention relates to a method for preparing three-dimensional graphene ultralight foams, and belongs to the technical field of novel materials and preparation thereof. The method comprises the following steps: carrying out irradiation treatment on a graphite oxide and acrylamide mixed water solution by utilizing the characteristics of high energy and strong penetrating power of gamma rays so that the graphite oxide contained in the graphite oxide and acrylamide mixed water solution is subjected to intercalation grafting, stripping and cross-linking, and then preparing compound three-dimensional graphene foams through one step; and directly carrying out freeze-drying and high-temperature heat treatment on the compound three-dimensional graphene foams to obtain the three-dimensional graphene ultralight foams. The method disclosed by the invention solves the problems of complex preparation process, high toxicity, high cost and the like of the three-dimensional graphene ultralight foams, has the advantages of simple preparation process, short period, high yield, low cost, environmental friendliness and the like and can realize the industrialized batch production.