A method for making a separator of a lithium ion battery is provided. In the method, a modifier, and a porous membrane are provided. The modifier is a mixture of a phosphorus source having a phosphate radical, a trivalent aluminum source, and a metallic oxide provided in a liquid phase solvent. The modifier is coated on a surface of the porous membrane to form a coating layer. The coated porous membrane is dried to form a modifier layer disposed on the surface of the porous membrane.