The invention belongs to the technical field of lithium ion battery materials, in particular to a composite film of hollow open microspheres assembled by carbon substrate loading nanosheets, and a preparation method thereof and application thereof. The composite film is a three-dimensional multi-stage structure assembled by the hollow-opened nickel-cobalt-manganese oxide microspheres as nanosheets. The composite film obtains a mixed solution of the hollow-opened nickel-cobalt-manganese oxide microspheres assembled by nanosheets and graphene through the two-step hydrothermal reaction; and carbon nanotubes are added to perform vacuum-filtering to obtain a flexible film with the hollow-opening microspheres inlaid into the three-dimensional porous carbon nanotube/graphene conductive network. The microsphere structure combines the advantages of the nanosheet and the hollow sphere structure, avoids the easy accumulation of the nanosheet, and improves the contactable active surface area. Themicrospheres are highly porous and open. Both ends of the nanosheet can be exposed to the pores of the stored electrolyte to provide more active surfaces and ion transport paths inside and outside thesphere.