The sulfur-free exfoliated graphite(EG) was prepared by chemical oxidation method using the mixture of HClO4 and CH3COOH as reaction medium, the 100 orders natural flake graphite(NG) as raw material, and KMnO4 as oxidant. The effects of influence factors on the exfoliation volume(EV) were analyzed by single factor experiments and orthogonal experiments, the structure of NG and EG were characterized by X-ray diffraction. Results show that the optimum preparation conditions were as follows: graphite(g): KMnO4(g): the mixed acid (mL) = 1:0.35:4, HClO4 (mL):CH3COOH (mL) = 3:1, reaction temperature at 35°C for 90 minutes, the maximum EV of up to 500mL/g. The influence factors are governed the descending order: the volume ratio of HClO4 :CH3COOH, reaction temperature, the dosage of mixed acid, the dosage of KMnO4, reaction time.