The invention discloses a thin film transistor, a preparation method thereof, an array substrate and a display device. The thin film transistor, the preparation method thereof, the array substrate and the display device resolve the problems that an electrode or a wire made of Cu causes poor or abnormal display of the display device due to high diffusion capacity or easy oxidation. The thin film transistor comprises a grid electrode made of pure copper, a grid electrode insulation layer, an active layer, a source electrode, a drain electrode and a copper alloy film layer, wherein the copper alloy film layer is arranged on the surface, facing the active layer, of the grid electrode. The thin film transistor, the preparation method thereof, the array substrate and the display device have the advantages that the copper alloy film layer is arranged on the surface, facing the active layer, of the grid electrode of the thin film transistor, accumulation of alloy elements at the crystal boundary, the crystal surface and the surface can prevent diffusion of copper atoms towards the active layer, the oxidation resistance of the grid electrode is improved, and therefore the problem of poor or abnormal display of the display device caused when the thin film transistor fails or the display device has a Mura defect is avoided.