The invention discloses a preparation method of a new nanometer composite adsorption material. The preparation method comprises the specific steps of, adding a certain amount of concentrated sulfuric acid in a beaker of 500 mL, in an ice-bath condition, slowly adding potassium permanganate and sodium nitrate, performing full stirring to dissolve potassium permanganate and sodium nitrate, adding expandable graphite while performing uniform stirring, and keeping the beaker in a refrigerator at 273 K for 24 h; and heating the beaker in an oil-bath pot to 308 K, maintaining the temperature while performing stirring for 30 min, slowly dropwise adding deionized water while performing stirring and raising the temperature to 371 K, and maintaining the temperature at 371 K while performing stirring for 15 min. According to the preparation method, agar is taken as a carrier to prepare active carbon/oxidized graphene/agar aerosol. Efficient adsorptive property of active carbon and oxidized graphene is fully developed, easy agglomeration of oxidized graphene is prevented, oxidized graphene with small size is hard to remove from a water body, and the problem of secondary micro-pollution to the water environment is solved.