The invention discloses a separator for a lithium ion battery and belongs to the technical field of lithium ion batteries. The separator comprises a polymer microporous film substrate, wherein the surface of the substrate and the internal wall of a micropore duct of the substrate are coated with nano-oxide layers. The preparation method comprises the following steps: (1) performing oxidation treatment on the polymer microporous film with an acid solution, then washing the film successively with an alkali solution and deionized water and drying the film; (2) putting the film on an atomic layer deposition apparatus or a continuous atomic layer deposition device, then vacuumizing, heating to 50-100 DEG C and keeping for 1-30min; (3) introducing trimethylaluminum as a first precursor; (4) adding nitrogen or inert gas for purging; (5) introducing a second precursor; (6) adding nitrogen or inert gas for purging; (7) repeating steps (3)-(6); and (8) performing heat setting treatment. The separator disclosed by the invention is small in thickness and heat shrinkage, and is good in wetting property with electrolyte. The preparation process temperature is low, a formed coating is dense, and the thickness of the coating can be precisely controlled.