Abstract: The traditional methods for preparing expanded graphite (EG) often require harsh conditions and take a long time resulting in a high-cost and severe environmental pollution. A simple, energy-saving and efficient route for preparing EG was developed, in which flake graphite was exfoliated with K₂S₂O₈ under concentrated H₂SO₄ at 80 ℃ for a few minutes. The microstructures, morphology and functional groups of the EG were characterized by XRD, SEM, FT-IR and Raman spectroscopy, and the electrical conductivity was measured by a four-point probe method. Results indicated that the flake graphite transformed into a worm-like structure after expansion with slight damage to the graphite sheets. A maximum expanded volume of 150 mL/g was obtained under the optimal conditions of 80 ℃ for 5 min with mass ratios of graphite to K₂S₂O₈ of 1∶7 and graphite to concentrated H₂SO₄ of 1∶20. The electrical conductivity of the flexible graphite film prepared by rolling from the optimized EG reached 5.47×10⁴ S/m. The mild oxidation and oxygen released by the decomposition of K₂S₂O₈ under the acidic conditions are responsible for the efficiency of the method, which is promising for the mass production of EG due to its simplicity, low cost and low environmental impact.