NOVELTY - An aluminum oxide ultra thin film is prepared by providing trimethylaluminum, nitrogen or argon, oxygen with 99.9999% purity and substrate, e.g. silicon chip, lanthanum nickelate, indium oxide or platinum sheet; and rinsing substrate by absolute ethyl alcohol, drying by argon, arranging on a sample tray, and feeding into a vacuum reaction cavity of atomic layer deposition system.    USE - Method for the preparation of aluminum oxide ultrathin film (claimed) useful in semiconductor, integrated ferroelectrics, optical plating or solar cell.    ADVANTAGE - The method has low reaction temperature, simple generation condition, high repeatability, no impurity and high control.    DETAILED DESCRIPTION - Preparation of aluminum oxide ultra thin film comprises:    (A) providing trimethylaluminum, nitrogen or argon, oxygen with 99.9999% purity and substrate, e.g. silicon chip, lanthanum nickelate, indium oxide or platinum sheet; and    (B) rinsing the substrate by absolute ethyl alcohol, drying by argon, arranging on a sample tray, feeding into a vacuum reaction cavity of atomic layer deposition system, evacuating until vacuum degree is 1-3 MPa, holding the substrate fixed on sample tray of vacuum reaction cavity at room temperature, freezing a source bottle contained with trimethylaluminum, holding at 15-20 degrees C, mixing oxygen and nitrogen with volume ratio of 95:5, generating ozone by an ozone generator, introducing nitrogen or argon into deposition system, holding air pressure intensity of reaction cavity and middle space at 1-3 or 6-15 MPa, where each generation cycle is composed of four gas pulses, e.g. trimethylaluminum pulse of 0.1 seconds, transferring nitrogen or argon into reaction cavity as gas carrier, where carrier gas flow is 150 standard cubic centimeters/minute (sccm), washing the cavity by nitrogen or argon for 2-30 seconds with flow of 150 sccm, at ozone pulse of 3 seconds, where the ozone flow is 200-300 sccm, transferring nitrogen or argon into reaction cavity as gas carrier, where carrier gas flow is 150 sccm, and washing the cavity by nitrogen or argon for 6 seconds with flow of 150 sccm; the aluminum oxide ultrathin film material by self-limiting absorbing effect, and the thickness of film is 35.33 nm when the cycle time is 250 seconds, and is 70 nm when the cycle time is 500 seconds.