The invention relates to the field of lithium ion batteries, and discloses a preparation method of a composite diaphragm for lithium ion batteries and a lithium ion battery containing the composite diaphragm. The preparation method of the composite diaphragm for lithium ion batteries comprises the following steps: polypropylene, ultra-high-molecular-weight polyethylene and pore-forming agent are mixed through a mixer, and the mixture is extruded to form diaphragms after drying; one diaphragm is longitudinally stretched to form a hole; the other diaphragm is transversely stretched to form a hole; the two diaphragms are composited by hot pressing; heat setting is carried out; and winding is carried out. The lithium ion battery containing the composite diaphragm for lithium ion batteries comprises a positive electrode, a negative electrode and electrolyte. The positive electrode uses carbon-coated aluminum foil as a current collector, and the negative electrode uses carbon-coated copper foil as a current collector. The conductive agent in the coatings of the carbon-coated aluminum foil and the carbon-coated copper foil is carbon black modified by polyvinylpyrrolidone grafting. The diaphragm longitudinally stretched to form a hole and the diaphragm transversely stretched to form a hole are composited into the composite diaphragm by hot pressing, so that the mechanical strength of the diaphragm is improved, and the self-discharge rate of the battery is reduced.