The invention belongs to the field of electrochemistry battery, and more specifically relates to a black phosphorus nanosheet-graphene composite film anode used for flexible lithium ion batteries, and a preparation method thereof. According to the preparation method, high purity and large scale black phosphorus blocks are synthesized via mineral substance assistant vapor transporting method with high efficiency; a large amount of clean and high quality black phosphorus sheets are prepared in water via ultrasound treatment; the clean and high quality black phosphorus sheets and high conductive graphene nanosheets prepared via intercalation stripping method are subjected to mixing ultrasonic dispersion; and flexible high-strength laminated composite film is prepared via vacuum filtration. The preparation process of the flexible high-strength laminated composite film is simple and controllable; large scale low cost preparation can be realized; black phosphorus nanosheets are high in capacity, graphene is high in electrical conductivity, and the black phosphorus nanosheets and graphene both possess two-dimensional structures high in flexibility and strength, so that problems the black phosphorus is poor in electrical conductivity and stability are solved, lithium ion battery capacity, cycling stability, and entire energy density are increased, and lithium ion battery electrode integrated flexible design is realized.