This invention discloses a method for preparing swelled graphite as high fire retardant. Recently, chemical oxidation method is the main method for producing the same, with the lamination-inserting agent, selected from: sulfuric acid, nitric acid or phosphoric acid. Lately, people pay more attention to swelled graphite with low sulphur or sulphur-free. In the inventive method, mixture solution ofsulfuric acid and nitric acid or concentrated sulfuric acid is used, and used is oxidized natural graphite containing solid strong oxidant with its content of 0.5-50% of the mass of the graphite. Thereaction temperature is 0-60deg.C, reaction time is 10-240 min, so obtained is primary product. It is then reacted in phosphoric acid solution containing poly-ammonium-phosphate as lamination-inserting agent, reacting at temperature of 0-60deg.C for 0.5-24hrs., so obtaining the inventive product. Said product can be used as fire retardant used in wide fields such as plastics, coatings and plasticfoam.