A thin-film transistor (TFT) and a manufacturing method thereof, an array substrate and a display device are provided. The TFT includes: a base substrate; a gate electrode and a gate insulating layer, disposed on the base substrate; and an active layer, wherein the gate insulating layer is disposed between the active layer and the gate electrode; the active layer includes a channel region and a doped region disposed on at least one side of the channel region; and the gate insulating layer is provided with a protrusion which is disposed between the doped region and the gate electrode.