The invention provides a lithium ion storage battery diaphragm. The diaphragm is a porous polyimide film and has a thickness of 5-50 microns. The difference between the largest pore diameter and the smallest pore diameter of the porous polyimide film is 0.002-0.1 microns. The porosity of the diaphragm is 30-60%. The invention also provides a method for preparing the lithium ion storage battery diaphragm. The lithium ion storage battery diaphragm of the invention causes the lithium ion storage battery to have a good security and to accord with the standard of lithium ion storage battery diaphragm.