According to embodiments of the invention, a thin film transistor (TFT), a manufacturing method of the TFT, an array substrate and a display device are provided. The manufacturing method of the TFT comprises: forming a gate electrode on a substrate; forming a gate insulating layer on the substrate formed with the gate electrode; forming an oxide semiconductor active layer, an etch stop layer and a source/drain electrode on the gate insulating layer, wherein the etch stop layer is obtained by an oxidation treatment.