NOVELTY - A multifunctional composite film has a high-strength polyfluortetraethylene substrate film (3) as middle layer, polyolefin film (2) as upper layer film and a polyolefin film (4) as lower layer film, which are adjacent to high-strength polyfluortetraethylene substrate film (3). The outer layers of the polyolefin film (2) and the polyolefin film (4) are polyvinylidene fluoride-hexafluoropropylene film (1) and polyvinylidene fluoride-hexafluoropropylene film (5), respectively.    USE - Multifunctional composite film is used for separator for lithium-ion battery.    ADVANTAGE - The separator containing the multifunctional composite film has improved thermal safety characteristics and soaking characteristics.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for preparation of multifunctional composite film, which involves pre-processing high-strength polyfluortetraethylene substrate film, preparing polyolefin films (2) and (4) solution by adding polyolefin, organic solvent and pore-forming agent in 20-60 degrees C constant temperature water bath under stirring, forming polyolefin films (2) and (4) on pre-processed high-strength polyfluortetraethylene substrate film by impregnation, spraying or coating, vacuum drying, extracting pore-forming agent using extracting agent to obtain polyolefin/polytetrafluoroethylene/polyolefin composite film, dissolving vinylidene fluoride and hexafluoropropylene copolymer and polymer in organic solvent, then adding plasticizer, heating to certain temperature, then adding initiator, stirring, mixing uniformly to obtain polyvinylidene fluoride-hexafluoropropylene film liquid, applying obtained liquid by impregnation, spraying or coating on composite film, drying at 25-10 degrees C and crosslinking to obtain multifunctional composite film.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the multifunctional composite film.    Polyvinylidene fluoride-hexafluoropropylene films (1,5)    Polyolefin films (2,4)    High-strength polyfluortetraethylene substrate film (3)