The invention provides a method for synthesizing polyaniline nano fibers and a reduced graphene oxide composite film under the condition of no other modifiers by using the combination of a chemical synthesis and hydrothermal treatment. The method mainly comprises the following process steps: step one, preparing a uniform and stable graphene oxide suspension; step two, carrying out in-situ polymerization in the graphene oxide suspension under a certain reaction condition to obtain the polyaniline nano fibers; step three, preparing a formed film by utilizing vacuum suction filtration; and step four, carrying out hydrothermal treatment. The composite film with good flexibility, stability and electrochemical performance is prepared through optimizing the experimental conditions, and the preparation method has a wide application prospect in the field of energy sources and the field of other electronic devices.