The invention discloses a lithium-ion battery separator subjected to composite coating treatment and a preparation method thereof. The lithium-ion battery separator comprises a basement membrane, a polymer coating coated at one side of the basement membrane, and an inorganic particle coating coated at the other side of the basement membrane; the polymer coating is composed of an aramid coating coated on the basement membrane and a polyvinylidene fluoride coating coated on the aramid coating, and the thickness of the polymer coating is 0.3 to 5 [mu] m; and the inorganic particle coating is composed of inorganic particles of different particle sizes and a binding agent, and the thickness of the inorganic particle coating is s 0.5 to 5 [mu] m. The lithium-ion battery separator provided by the invention has the characteristics of being excellent in mechanical property and heat resistant property, having good wettability and solution preserving property to electrolyte, and excellent in cycle performance and high in safety performance.