The invention relates to metal surface protection, and in particular relates to a method for preparing a composite graphene / silane film on the surface of metal. The method overcomes the defect that the binding strength of a graphene film and the metal is weak; a silane coupling agent with two different chemical functional groups is introduced as a transition film layer to prepare a composite film, and the composite film is prepared from graphene and the silane coupling agent. The chemical bond connection between the metal and graphene is achieved through the silane coupling agent, and the binding strength of graphene and the metal is improved; the prepared composite film is long in service life, and the anti-corrosion property of the metal is effectively improved.