The present disclosure provides a thin film transistor and its manufacturing method, an array substrate, a display device. The thin film transistor includes a gate electrode, a gate insulating layer, an active layer, a source electrode and a drain electrode. The source electrode and the drain electrode are formed above the active layer and located at a first end and a second end of the active layer which are opposite to each other, respectively. The drain electrode completely covers the second end of the active layer.