The embodiment of the invention provides a thin film transistor and a preparation method thereof, an array substrate and a display device, and relates to the technical field of display, being capableof solving the problem that the preparation process of an active layer is cumbersome. The process of forming the active layer on a substrate comprises the steps of forming a semiconductor pattern on the substrate, wherein the semiconductor pattern comprises a channel region; forming a barrier layer on the semiconductor pattern; injecting ions into the semiconductor pattern for the first time at afirst angle, and separately forming a first doped region on each of the first and second sides of the channel region; injecting the ions into the semiconductor pattern for the second time at a secondangle, and forming a second doped region on the first side of the channel region; and injecting the ions into the semiconductor patterns for the second time at a third angle, and forming the second doped region on the second side of the channel region, wherein the ion concentration of the first-time ion injection is more than that of the second-time ion injection, all the first angle, the second angle and the third angle are included angles between ion beams and the substrate; and the second angle and the third angle are less than the first angle.