The invention discloses a method for preparing a graphene oxide. The graphene oxide is prepared from a graphite raw material in a mode that mechanical action and oxidation reaction are combined. The method comprises the following steps of: mixing an oxidant with the graphite raw material, subsequently preparing the graphene oxide by employing mechanical action, adding the oxidant after the graphite raw material is subjected to mechanical action so as to prepare the graphene oxide, and adding the oxidant after the graphite raw material is subjected to mechanical action so as to carry out oxidation reaction and prepare the graphene oxide. According to the method, strong acids and strong oxidants required in a chemical oxidation method are abandoned, so that the introduction of byproducts and impurities is greatly reduced, the separation and the purification are convenient, the process procedures are reduced, the treatment time of a mechanical milling method is shortened, the energy consumption is reduced, and the yield is greatly increased. With the combination of the two methods, the graphene oxide can be prepared at one step within the shortest time, the yield is greater than 20%, the graphite oxide can be stably suspended under a centrifugal condition of 16,000rpm without ultrasound, and the method has the characteristics of rapidness, simplicity, convenience, environmental-protection and efficiency.