A graphite oxide or graphene preparation method includes providing a plasma electrolytic apparatus, where an electrolytic solution is provided and a graphite electrode is configured as a cathode of the plasma electrolytic apparatus; and providing a cathodic current so as to initiate a plasma electrolytic process at the graphite cathode to obtain graphite oxide or graphene. The graphite oxide can be synthesized through plasma electrolytic processing at relatively low temperature under atmospheric pressure within a very short period of time, without the need for concentrated acids or strong oxidizing agents. The present invention may prepare graphite oxide with plasma electrolytic process directly from graphite, without requiring any prior purification. This plasma electrolytic process of the present invention is quite promising and provided with advantages such as low cost, simple setup, high efficiency, and environmental friendliness.