Abstract: Abstract: Expanded graphite was prepared by a chemical method using natural flake graphite, concentrated nitric acid and acetic anhydride as intercalation agent, and potassium permanganate as oxidant. The conditions for preparing the expanded graphite was optimized by an orthogonal test method and factors affecting expanded volume and intercalation mechanism were obtained. Results showed that sulfur-free and expanded graphite of 478 mL/g could be produced under the optimum conditions with the weight ratio of natural flake graphite : nitric acid : acetic anhydride : potassium permanganate being 1 : 0.7 : 1.5 : 0.4, reaction time of 90 min, and reaction temperature 30-40℃. The descending order of affecting factors is KMnO4, nitric acid, acetic anhydride and reaction time.