The invention provides a thin film transistor. The thin film transistor comprises an active layer, a gate insulated and spaced with the active layer, an insulating layer, a source contacted with the active layer through a source via hole penetrating through the insulating layer as well as a drain contacted with the active layer through a drain via hole penetrating through the insulating layer, wherein the source via hole forms a stepped hole, and the drain via hole forms a stepped hole. The invention further provides an array substrate used for a display panel, the display panel and a manufacturing method of the thin film transistor. Compared with the prior art, a mask plate is used one less time less when the thin film transistor is manufactured, and accordingly, the manufacturing cost of the thin film transistor is reduced.