The invention belongs to the display technical field, and relates to a thin film transistor (TFT) and a preparation method, an array substrate and a preparation method, and a display device. The TFT comprises a grid, a gate insulation layer, a source, a drain electrode and an active layer, wherein the drain electrode, the active layer and the source are successively laminated; the orthographic projection area of the active layer on the plane parallel with the drain electrode is the same with the area of the drain electrode; the orthographic projection area of the source on the plane parallel with the drain electrode is less than the area of the drain electrode; the gate insulation layer is at least partially in contact with the active layer; the grid is arranged outside the gate insulation layer and covers the side surface and partial top surface of the gate insulation layer away from the source. The TFT can obtain high starting current under low resistance in an open state, minimize off-current in a closure state, and effectively inhibit off-current.