The invention relates to a microwave chemical preparation method of ultrafine-flake sulfur-free expanded graphite. The technical scheme is that the method comprises steps as follows: graphite is mixed with a composite oxidant and an intercalator, a mixture is subjected to a reaction for 10-100 min at the temperature of 20-100 DEG C, a product is washed with deionized water until pH is neutral, dried and then subjected to microwave processing for 5-60 s under the power of 500-1,200 W, and a target product is obtained. According to the method, natural ultrafine-flake graphite with the average particle size smaller than 30 mu m is taken as a raw material, the sulfur-free expanded graphite with an expanded multiplication rate of 24 ml/g can be prepared from the raw material with the microwave chemical preparation method, and the method is also suitable for common natural flake graphite. The method adopts a simple technological process, is easy to operate and low in cost, the raw material is easily available, and no additional elemental sulfur is dragged in.