The invention discloses a preparation method of large-size graphene oxide or graphene. The preparation method mainly utilizes graphite as a raw material and comprises that 1, under the action of an intercalator and an expanding agent, graphite fully releases interlayer spaces so that interlayer interaction is reduced and a graphene aggregate is obtained, 2, the graphene aggregate is oxidized by an oxidizing agent, and then the oxidized graphene aggregate is peeled in water under gentle mechanical action so that a large area of a dispersion liquid of graphene oxide is obtained, and 3, the peeled graphene oxide is reduced by a reducer or heat treatment so that high conductivity graphene is obtained. The preparation method utilizes cheap raw materials, has a simple process, can be controlled easily, realizes fast and high efficiency solid-liquid separation, is convenient for industrial large scale production, prevents graphene crystal structure destroy caused by high energy supersonic wave, high speed shearing or fluid crushing. The obtained graphene has a large size and high conductivity and can be popularized and used in fields of high efficiency heat management, flexible display, energy conversion and storage.