The invention provides a preparation method of graphene. The preparation method comprises the following steps: firstly, performing a reaction on graphite, chromium trioxide and concentrated hydrochloric acid to obtain an intercalated graphite compound; then mixing the intercalated graphite compound obtained in the previous step with hydrogen peroxide to obtain expanded graphite; and finally, peeling the expanded graphite obtained in the previous step in an aqueous phase to obtain the graphene. In order to overcome the defect of high temperature and high pressure of an existing conventional graphene liquid phase peeling method, a graphene preparation method of low-temperature intercalation, low temperature expansion and aqueous phase peeling is creatively achieved by optimizing and combining raw materials and refining the reaction steps by means of a low-temperature constant-pressure liquid phase peeling method. Therefore, the production cost is lowered, the environmental pollution caused by an organic solvent is also avoided, and the dispersing speed of graphene is also increased.