To improve the safety performance of the battery diaphragm, polyethylene diaphragm is used as base membrane, polyacrylate and alumina are used as coating solution to develop one side high purity alumina coating diaphragm. Puncture strength of diaphragm is measured by CCY-02 puncture force tester, tensile strength of diaphragm is measured by XLW intelligent electronic tensile force tester, permeability of diaphragm is measured by TQD-G1 air permeability tester, liquid absorption rate of diaphragm is calculated by weighing; thermal shrinkage of diaphragm is detected by RSY-R1 thermal shrinkage tester at room temperature to study the mechanical properties, thermal stability, air permeability and moisture content of diaphragm by introducing coating layer. The microscopic characterization of the composite diaphragm was studied by FTIR, EDS, and SEM. After the high purity alumina coating was introduced, the comprehensive performance of the composite diaphragm is obviously improved:When the amount of polyacrylate in the coated solution is 6% and the amount of pure alumina is 40%, the overall performance of the diaphragm is optimal, puncture strength reached 785 g, transverse tensile strength of 158.6 MPa withthe longitudinal tensile strength of 121.7 MPa, air permeability of 263 s/100 mL, the suction rate of 300%. Under vacuum at 130 ℃ for 1 h, the transverse thermal contraction is 2.18%, vertical collection shrink is 0.83%. The results of analysis the component and microscopic characterization of the composite diaphragm show that the surface of the modified membrane forms a well-distributed microporous structure, which promotes the transfer of lithium ions between positive and negative poles. Because of the physicochemical characteristics of alumina, the heat resistance of lithium battery diaphragm is improved. Therefore, the high purity alumina composite diaphragm plays a positive role. In general, the comprehensive performance of lithium ion battery diaphragm is greatly improved after the introduction of high purity alumina coating.