The invention discloses a method for preparing nano graphene powder by plasma chemical vapor synthesis, which comprises the following steps: by using nitrogen as a working medium, the raw material methane is ionized into carbon ions and hydrogen ions under the ionization action of plasma arc, wherein the carbon ions are rearranged into a network in the reactor to generate complete-structure graphene powder, the hydrogen ions are combined into hydrogen gas to be discharged, and the graphene powder is led by the air flow to enter a powder collector. The graphene prepared by the method has the advantages of lower layer number (2-10), complete structure, large single-layer diameter, low oxygen content, no sulfur and low metal impurity content, and can implement continuous industrialized production.