Embodiments of the present invention disclose a thin film transistor and a manufacturing method thereof, an array substrate, and a display device, which relates to the field of display technology, and solves the problem that the adhesion of the electrode thin film with the adjacent thin film layer in the thin film transistor of the prior art is relatively bad. More specifically, an embodiment of the present invention provides a thin film transistor, comprising a gate, a source, a drain and a buffer layer, the buffer layer is located at one side or two sides of the gate, the source or the drain, the material of the buffer layer is a copper alloy material, the copper alloy material contains nitrogen element or oxygen element, the copper alloy material further contains aluminum element.