A method for manufacturing a thin-film transistor (TFT), an array substrate and a display device are disclosed. The manufacturing method includes: forming a photoresist layer provided with a completely retained region, a partially-retained region and a completely removed region on a metal film by a half-tone mask process; forming a source/drain metal layer by etching the metal film under the cover of the photoresist layer; removing the photoresist layer in the partially-retained region; forming an active layer by patterning the semiconductor film; and removing residual photoresist layer.