The work aims to improve the properties of the inner film material for lithium battery soft packaging and meet the packaging requirements. Boron nitride (BN) was treated with (3-aminopropyl) triethoxysilane (KH550) and then added to PP/PP-g-GMA matrix to prepare the inner film material of lithium battery soft packaging. The effects of the coupling agent, filler dosage and process conditions on the strength, toughness and electrolyte resistance of the materials were investigated. The tensile strength and impact strength of KH550-BN composite were better than that of the pure BN system. When BN content was 50 g, the tensile strength and impact strength of KH550-BN composite reached 29.2 MPa and 6.15 kJ/m2, respectively 1.4% and 7.9% higher than that of pure BN system. After soaking in vinyl carbonate for 36 h, the composite still maintained high strength. The proposed composite has good toughness and electrolyte resistance, which provides a reference for the development of inner film of lithium battery soft packaging.