A method for fabricating an array substrate, an array substrate, and a display device are disclosed. The method includes forming a whole layer of opaque film on a substrate; treating the film to form a transparent region and an opaque region in the film, wherein the opaque region corresponds with a channel region of an active layer; and forming a thin film transistor on the film which has been treated. In the method, prior to forming the thin film transistor, the whole layer of opaque film is formed to comprise the transparent region and the opaque region. When other films are deposited on the whole layer of film, no difference in height occurs, and this further avoids various defects due to difference in height.