The invention belongs to the field of nanostructure materials, and specifically provides a method for preparing a flat barrier type TFT anodic aluminum oxide insulating layer. After the aluminum film is plated by magnetron sputtering, the invention directly performs secondary sputtering under vacuum conditions. A very thin layer of magnesium nitride, magnesium sulfide, magnesium oxide, calcium oxide, calcium peroxide, calcium nitride or calcium sulfide film is continuously plated on the surface of pure aluminum, which effectively prevents the freshly plated pure aluminum surface from contacting the air. The amorphous natural alumina film is rapidly formed by contact; and, the second sputtered magnesium nitride, magnesium sulfide, magnesium oxide, calcium oxide, calcium peroxide, calcium nitride or calcium sulfide film, contact with weak acid electrolyte Afterwards, the hydrolysis reaction produces a small amount of alkaline substances dissolved in the weakly acidic electrolyte, which will not affect the subsequent anodic oxidation process of aluminum; the present invention can not only obtain a highly smooth barrier-type anodized aluminum film, but also avoid the aluminum The risk of the film falling off instantly, and the process is simple and the cost is low.