An embodiment of the invention relates to a thin film transistor, a manufacturing method thereof, an array substrate and a display device. The thin film transistor comprises a source electrode, a drain electrode and an active layer, wherein the source electrode, the drain electrode and the active layer are arranged on the same layer; the source electrode and the drain electrode are connected with the active layer through the side surface of the source electrode and the side surface of the drain electrode; the source electrode and the drain electrode are made of metal; and the active layer is made of a metal oxide semiconductor which corresponds to the material of the source electrode and the material of the drain electrode. By the thin film transistor, working procedures can be reduced, and the cost is also reduced.