The invention relates to a method for preparing graphite by compressing CO2 fluid under ultrasonic assistance. CO2 is filled to clearances between graphite layers, and graphite is stripped under the ultrasonic assistance to form the graphite. Compared with the prior art, the method has the advantages of mild experiment condition, short treatment time, low required pressure, high yield, good quality, environment friendliness and suitability for large scale production; and moreover, the method is a physical process, so the perfection of the grapheme is ensured.