The invention provides a heat-resisting lithium battery diaphragm and a preparation method thereof. The method comprises the following steps: (a) weighing a component A, which comprises the following materials: polyether polyol, a foaming agent, a catalyst, a stabilizer and a fire retardant in weight ratio, pouring the materials into stirring equipment and mixing the materials evenly; (b) weighing a component B MDI in the weight ratio, respectively feeding the weighed materials of the component A and the material of the component B into a material jar A and a material jar B of a polyurethane foaming machine, stirring the materials in the polyurethane foaming machine, filling the materials into a rectangular or square mold, and carrying out foaming molding; and (c) putting a pelletized product which is molded in a foaming manner on a special foam horizontal cutting machine, cutting the pelletized product into slices according to the thickness requirement, and carrying out punch forming on the slices according to the size specification requirements, wherein the thicknesses of the slices are 5-41 microns; the porosity is 70%-75%; and the air permeability is 15-30 sec / 100cc. The heat-resisting lithium battery diaphragm has the advantages of high porosity, low thickness, high absorption amount, high ion permeability, low resistance, good compressive property, stable electrochemical properties, simplicity in manufacturing process, low cost, high quality and the like.