The invention discloses an aluminum fluoride (AlF3)-modified composite membrane for a lithium ion battery and a preparation method thereof. The aluminum fluoride-modified composite membrane is characterized in that AlF3 ceramic particles are uniformly sputtered on an electrostatic-spinning polyvinylidene fluoride nanofiber membrane by a magnetron deposition technology to prepare a composite nanofiber membrane. By the aid of the magnetron deposition technology, AlF3 particles are deposited on the surface of a basement membrane to form a composite ceramic layer, so that on one hand, heat resisting performance of the membrane is improved, and safety of the lithium ion battery is improved; on the other hand, since Lewis acidity of AlF3 is capable of restraining electrolyte decomposition to a certain degree, cycling performance of the lithium ion battery is improved effectively. The aluminum fluoride (AlF3)-modified composite membrane for the lithium ion battery has the advantages that defects of infirmness, large thickness and the like of coating of a normal ceramic particle layer are overcome, remarkable effects in electrolyte decomposition restraining and cycling performance improvement are realized, and thereby being applicable to the field of lithium ion battery membranes.