The invention relates to the technical field of graphene and provides a preparation method of the graphene. Raw materials, potassium ferrate and aspartic acid are mixed before being ball-milled; after ball-milling is over, diluted hydrochloric acid is used for washing powder and conducting centrifugation until supernate is neutral, and then distilled water is used for washing and centrifuging; lastly, the powder is dried to constant weight. The preparation method of the graphene has the advantages that the method is simple and effective, the thickness of the prepared graphene is 1 layer or 2 layers, the structure is intact, and little damage is caused to the structure of the graphene; the adopted solvothermal method meets the requirements for efficient preparation of ferroferric oxide and compounding of graphene and ferroferric oxide; the adopted materials are clean, environmentally friendly and free of secondary pollution, and therefore the preparation method is an environment-friendly preparation route.