By using nature flake graphite of 180 μm as raw material，sulfurfree expandable graphite was synthesized via che mical oxidation method in the mixed acid（acetic anhydride phosphoric acid perchloric acid） and mixed solid assistant oxi dant（potassium permanganatepotassium dichromate） system.The optimum preparing conditions were as follows：m（flake graphite）∶V（mixed acid）∶m（potassium permanganate）∶m（potassium dichromate）=1（g）∶4（mL）∶0.1（g）∶0.1（g），reacted at 50 ℃ for 60 min and dried at 40 ℃ for 2 h.Under these conditions，the sulfurfree expandable graphite with an initial expansion temperature of 200 ℃ was obtained，meanwhile the maximum expanded volume of which could reach up to 665 mL/g when exfoliated at 500 ℃.The structure，morphology and functional groups of flake graphite，expandable graphite and expanded graphite were characterized by SEM，FT-IR，XRD and TGA.Results showed that perchloric acid，phosphoric acid and acetic anhydride successfully inserted into the graphite layers as a result of the expansion of interlaminar spacing of expandable graphite，and after puffing，sulfurfree expanded graphite was obtained，which had large expanded volume，high purity，rich space structure and the same slice layer crystal structure as flake graphite.