The invention discloses a preparation method of graphene oxide. A key of the technology is that by means of natural flake graphite as a carbon raw material, phosphoric acid and sulfuric acid as reaction mediums, and potassium permanganate and hydrogen peroxide as oxidizing agents, a reaction is carried out under normal pressure to prepare the target product. The method has the following advantages: (1) the raw materials have wide sources and low cost and are easy to obtain and prepare; (2) oxidizing effect is excellent, size of the product is uniform, yield is high, separation is simple and effect of saving energy and reducing emission is significant; and (3) the reaction is safe and stable and has no generation of sparkles and smokes, so that the method is easy to scale-up industrially.