The invention discloses a method for preparing supersize single-layer graphene oxide by virtue of a one-step method. The method is an improved Hummers preparation system. The method comprises the following steps: adding graphite, a template protecting agent and a strong oxidant into concentrated sulfuric acid or mixed acid of concentrated sulfuric acid, reacting at 10 DEG C below zero-40 DEG C for1-48 hours, and diluting concentrated sulfuric acid by virtue of deionized water; removing a residual high-valence oxidant with hydrogen peroxide, so as to obtain a bright yellow product; and finallycarrying out repeated centrifugation and washing on the obtained product until the pH value of the liquid reaches 5-7, so as to obtain supersize single-layer graphene oxide. The preparation process is simple and controllable and can be used as a special method for accurately preparing supersize single-layer graphene oxide with a diameter over 400 microns.