The present invention discloses a thin-film transistor (TFT), a manufacturing method thereof, an array substrate and a display device. The present invention is used for improving the electrical properties of the TFT and the image quality of the display device. The TFT provided by the present invention comprises: a gate electrode, a source electrode, a drain electrode, a semiconductor layer, a gate electrode insulating layer and a first metal barrier layer, which are disposed on a substrate; the gate electrode insulating layer is disposed between the gate electrode and the semiconductor layer; and the first metal barrier layer is disposed between the source/drain electrodes and the gate electrode insulating layer, and the first metal barrier layer is arranged on the same layer as the semiconductor layer and configured to prevent interdiffusion between the material for forming the source/drain electrodes and the material for forming the gate electrode.