NOVELTY - An insulating film comprises a porous substrate, and coating provided on a surface of the porous substrate, where the coating comprises polyurethane compound (I), and an end segment A of the polyurethane has terminal isocyanate groups and an end segment B has amino group.    USE - Insulating film for a lithium ion secondary battery (claimed).    ADVANTAGE - The insulating film has high physical mechanical property, high temperature property and adhesive property and improves safety of the lithium ion secondary battery.    DETAILED DESCRIPTION - An insulating film comprises a porous substrate, and coating provided on a surface of the porous substrate, where the coating comprises polyurethane compound of formula (I), and an end segment A of the polyurethane has terminal isocyanate groups and an end segment B has amino group.    R1 = monomers having terminal hydroxyl groups, 2C or more alkane, alkene, aromatic hydrocarbon, ether and/or ester;    R2 = monomers having 2C or more linear or cyclic diisocyanate;    R3 = monomers having 3C or more linear or cyclic acid with two or more hydroxyl groups;    R4 = monomers having 2C or more chain or cyclic diamines;    R5 = monomers having 6C or more linear or cyclic dicarboxylic acid, tetracarboxylic acid, monocarboxylic anhydride, dicarboxylic anhydride or benzene ring; and    m, n = 1.    INDEPENDENT CLAIMS are also included for the following:    (1) a method for preparing an insulating film, which involves adding organic solvent and catalyst with a temperature sensor, a reflux condenser tube, a stirring blade and three-necked reaction vessel, adding monomers to the vessel, stirring the monomers at room temperature and continuously stirring the reaction temperature to obtain-terminal isocyanate groups of a segment A, adding aprotic polar solvent to another three reactor with a temperature sensor, a reflux condenser tube, stirring blade and heating the mixture while stirring monomer and reacting with an amine to give the end of the segment B, adding the segment B slowly to the reaction vessel, controlling the reaction temperature and stirring to obtain the polyurethane, adding deionized water to the three sections of the reactor, stirring, and dissolving polyurethane in deionized water to adjust the viscosity of the solution to obtain a solid content i.e. to obtain a polyurethane coating, and spraying or dipping the resulting homogeneous polyurethane coating on a surface of the porous substrate and drying to obtain the insulating film    (2) a lithium ion secondary battery.