A method for preparing expandable graphite or graphene by utilizing waste cathode carbon formed in electrolytic aluminum production is disclosed. The method includes (1) a step of crushing, adding a strong oxidant and water into the crushed product, stirring, rolling or wet-grinding the mixture, and performing a hydrothermal oxidation reaction; (2) a step of controlling the temperature, pressurizing, feeding supercritical or liquid carbon dioxide, mixing the mixture by stirring the mixture, allowing the mixture to stand, adjusting the temperature and pressure, performing separation to obtain aqueous slurry and graphite oxide slurry fluid, and washing the graphite oxide slurry fluid with water to remove impurities; and (3) a step including 1) a manner of performing atomization, expansion and stripping to remove the carbon dioxide; or 2) a manner of adding a reactant for reduction, and then performing atomization, expansion and stripping to remove the carbon dioxide, or further performing denitrification and deoxygenation; or 3) a manner of adding a fluorinating agent, heating the mixture, performing a fluorinating reaction, and performing atomization, expansion and stripping to remove the carbon dioxide. Expanded graphite powder, graphene powder, denitrified deoxygenated graphene powder or fluorinated graphene powder which are prepared by the method have excellent properties. The method is simple, efficient, clean, free of secondary pollution, low in energy consumption and suitable for industrial production.