NOVELTY - Preparing graphene quantum dot, comprises (i) preparing the graphene sheet, (ii) dispersing graphene sheet in the mixed solvent of concentrated sulfuric acid and nitric acid, performing ultrasonic treatment, stirring to obtain the product dispersed in deionized water, and cooling to obtain the dispersion liquid containing graphene sheet, (iii) adhering dispersion liquid on the filtering film, redispersing, transferring into a microwave reactor, cooling and collecting the filtrate, and (iv) placing the filtrate in a dialysis bag, dialyzing, collecting the product, and drying.    USE - The graphene quantum dot is useful in bioluminescence marker and solar cell.    ADVANTAGE - The method has simple synthesizing technique, and has high efficiency. The graphene quantum dot has high purity, monodispersity, and small particle size; is soluble in water; and has strong fluorescence property.    DETAILED DESCRIPTION - Preparing graphene quantum dot, comprises (i) adding the graphite oxide in the glass tube, where the glass tube is set in the quartz tube and the quartz tube in a tubular furnace, increasing the temperature to 160-200 degrees C under the protection of argon atmosphere, maintaining the temperature for 2-4 hours, then naturally cooling to obtain the graphene sheet, (ii) dispersing graphene sheet in the mixed solvent of concentrated sulfuric acid and concentrated nitric acid, performing ultrasonic treatment for 8-10 hours, and stirring to obtain the product, which is dispersed in deionized water, and cooling to obtain the dispersion liquid containing graphene sheet, where each 40 ml of dispersion liquid containing the graphene sheet comprises 50 mg graphene sheet, 0-30 ml concentrated sulfuric acid, 10-40 ml concentrated nitric acid, and deionized water supplementing to 40 ml, (iii) adhering dispersion liquid containing graphene sheet on the filtering film, redispersing in deionized water, adjusting the pH to 8 using sodium hydroxide solution, then transferring the dispersion liquid into a microwave reactor of 150 W, where the working frequency is 40 Hz for 3 minutes, naturally cooling and collecting the filtrate with filter film, and (iv) placing the filtrate in a dialysis bag with molecular weight cut-off of 8000-140000 Da, dialyzing using deionized water for 3 days, and are fed once for every 12 hours with deionized water, collecting the product, and drying to obtain the product.