The invention discloses a graphene nanometer material preparation method, which comprises: (1) placing expandable graphite in a crucible, carrying out microwave treatment, adding the treated graphiteinto a dispersant, carrying out ultrasonic treatment to obtain a slurry, carrying out centrifugation on the slurry to divide the slurry into an upper layer and a lower layer, and removing the supernatant to prepare a graphene oxide; (2) dispersing p-aminobenzenesulfonic acid into an acidic solution to obtain a mixed solution, placing the mixed solution in an ice-water bath, stirring, adding a nitrous acid aqueous solution into the mixed solution in a dropwise manner to obtain a white precipitate, and carrying out filtration washing to obtain a p-aminobenzenesulfonic acid diazo salt; and (3) mixing the graphene oxide and the p-aminobenzenesulfonic acid diazo salt, carrying out stirring washing filtration under ice-water bath to obtain a sulfonic acid group-modified graphene oxide slurry, baking the sulfonic acid group-modified graphene oxide slurry until the slurry is dried, and finally crushing the sulfonic acid group-modified graphene oxide to obtain the sulfonic acid group-modified graphene oxide.