The present invention discloses a method for manufacturing a thin-film transistor, comprising the steps of: forming a semiconductor active layer, and a doped semiconductor active layer; forming a source-drain metal layer; forming a channel region; and implanting ions for lowering the TFT leakage current into the surface of the semiconductor active layer in the channel region via ion implantation after forming the channel region. The invention further relates to a thin-film transistor, a TFT array substrate and a display device. The invention has the following beneficial effects: by implanting ions for lowering the TFT leakage current into the channel region, the electrical performance of a TFT may be improved, and the thickness of a semiconductor active layer in a channel region may be changed controllably.