Recent research into graphene-based materials is largely focused on graphene quantum dots (GQDs) and their optical properties. A facile method has been developed to extract GQDs from reduced graphene oxide (RGO) by the ozonation pre-oxide method. The as-prepared GQDs, which were 2–5 nm in diameter, exhibited strong fluorescence activity ranging from 355 nm to 440 nm. The prepared GQDs possessed strong fluorescence with quantum yields from 3.18% to 9.48%. What's more, the fluorescence properties of the GQDs could be determined by tuning the pH of the ozonation system. We speculated the mechanisms of ozonation, thermal and hydrothermal treatment. We found that pyrocatechol could lead to fluorescence quenching of the GQDs, which might produce novel potential for the detection of targets.