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.