Disclosed in the present invention are a method for manufacturing a thin-film transistor, an array substrate, and a display device. The method includes: forming a buffer layer on a substrate; forming a polysilicon layer on the buffer layer; performing a patterning process on the polysilicon layer, to form an active layer; depositing a gate insulating layer on the active layer; depositing a gate metal layer on the gate insulating layer, and performing dry etching on the gate metal layer by using the patterning process and by using a gas containing CO as an etching gas, to form a gate; performing ion implantation on the active layer by using the gate as a mask, to form a source region and a drain region; and depositing a passivation layer on the gate, forming through holes in the gate insulating layer and the passivation layer, and manufacturing a source and a drain.