There is provided a method and an apparatus for preparing graphite oxide. In accordance with the present disclosure, when an oxidizer is added by stages into a sequencing batch oxidizer mixer connected in multiple stages at a constantly low temperature, the oxidizer is mixed therein while suppressing an oxidation reaction. During an oxidation reaction of graphite, in a risky range of overheating and explosion, a tube type reactor equipped within a heat exchanger is used to accurately control a local temperature and the oxidizer can permeate between layers of the graphite with increased efficiency under ultrasonication. In an additional reaction range out of the risky range, a continuous stirred tank reactor is used to mature the oxidation reaction, so that a risk of explosion during manufacturing of graphite oxide can be reduced and a great amount of graphite oxide can be mass-produced economically.