The invention relates to the technical field of thin film transistor display, and discloses a thin film transistor, a manufacturing method thereof, an array substrate and a display device. The thin film transistor comprises gate electrodes, an active layer, source/drain electrodes, and a gate insulation layer positioned between the gate electrodes and the active layer, wherein each gate electrode comprises a gate electrode metal layer and a first group of protective layers; the first group of protective layers are positioned between the gate electrode metal layers and the gate insulation layer for isolating the active layer from the gate electrode metal layer; and the gate electrode metal layers are made of copper or copper alloys. With the adoption of the technical scheme, the gate electrode metal layer is isolated from the active layer by the first group of protective layers, so that the active layer is prevented from being polluted due to copper diffusion, and the yield of a product is increased greatly.