The present disclosure provides a TFT, its manufacturing method, an array substrate and a display device. The method includes steps of: forming a pattern of a gate electrode on a base substrate; forming a gate insulation layer with an even surface; forming a pattern of a polysilicon semiconductor layer; and forming patterns of a source electrode and a drain electrode. The step of forming the pattern of the polysilicon semiconductor layer includes: crystallizing the amorphous silicon layer, so as to form the polysilicon semiconductor layer.