The invention discloses a bacterial cellulose / manganese dioxide / polypyrrole composite film and a preparation method thereof. The composite film is prepared by soaking bacterial cellulose in alkali, performing impurity removal and pretreatment, adding potassium permanganate and pyrrole, performing oxidization and reduction to obtain manganese dioxide and polypyrrole, and finally performing suction filtration. According to the bacterial cellulose / manganese dioxide / polypyrrole composite film, polypyrrole which is high in air stability, can be easily prepared into a film in an electrochemical way and is low in poison is utilized, and manganese dioxide which shows high capacitive property in neutral aqueous electrolyte under a wider potential window, and high-strength material bacterial cellulose are compounded, so that the method is simple and feasible. Polypyrrol is loaded on bacterial cellulose, so that the conductivity of the film is improved; meanwhile, manganese dioxide has high capacitive property, so that the electrical property of the composite film is further improved. The bacterial cellulose / manganese dioxide / polypyrrole composite film can be applied to the fields of lithium ion battery cathode materials, supercapacitors, electronic devices and biological sensor wrapping materials.