Method for preparing graphene sheet

The invention discloses a method for preparing a graphene sheet. The graphene sheet is prepared through the following steps: (1) expansible graphite is subjected to thermal expansion treatment so as to form wormlike graphite, the wormlike graphite is then soaked in alkaline solution, is subjected to suction filtration, is washed and then is put in the solvent to be subjected to ultrasonic dispersion, so that a micro-nano graphene sheet is obtained; (2) the micro-nano graphene sheet which is obtained in the step (1) is dispersed in inorganic strong protonic acid to be soaked, is subjected to suction filtration, is then transferred into H2O2 solution to be soaked and is subjected to suction filtrating, washing and drying; (3) the micro-nano graphene sheet which is treated through the step (2) is transferred into the solvent to be dispersed, is enabled to stand at constant temperature, and is then subjected to suction filtrating or drying to be secondarily expanded; and (4) the micro-nano graphene sheet which is