The invention relates to a lithium battery diaphragm and a preparation method thereof. The lithium battery diaphragm has a multilayer structure, an organic polymer nuclear pore membrane serves as a base membrane, modified graphene oxide membranes are compounded on two sides of the base membrane, and a closed layer membrane is compounded on the outer layer of the graphene oxide membrane; the organic polymer nuclear pore membrane adopts a polyester (PET) nuclear pore membrane, a high density polyethylene nuclear pore membrane or an ultra-high molecular weight polyethylene nuclear pore membrane. The battery diaphragm disclosed by the invention has uniform pore structure and pore distribution as well as high porosity; due to the addition of the modified graphene oxide material, the hydrophilic performance of the membrane layer is improved, the environment adaptability of the diaphragm is improved, the resistance is reduced, and the battery efficiency is improved. Moreover, using metallic materials is avoided, and environmental hazards caused by the electrolyte membrane are reduced.