A thin-film transistor (TFT), a manufacturing method thereof, an array substrate and a display device are disclosed. The method for manufacturing the a TFT comprises the step of forming a gate electrode, a gate insulating layer, an active area, a source electrode and a drain electrode on a base substrate. The active area (4) is made of a ZnON material. When the gate insulating layer is formed, a material for forming the gate insulating layer is subjected to control treatment, so that a sub-threshold amplitude of the TFT is less than or equal to 0.5 mV/dec. The manufacturing method reduces the sub-threshold amplitude of the TFT and improves the semiconductor characteristic of the TFT.