An anti-diffusion layer, a preparation method thereof, a thin-film transistor (TFT), an array substrate and a display device are provided, involve the display device manufacturing field and can resolve problem that a high atmosphere temperature is need in process of preparing a tantalum dioxide anti-diffusion layer by PVD or CVD, which causes the gate electrode to volatilize and affect the performance of a display device. The method for preparing the anti-diffusion layer comprises: placing a conductive base (1) and a cathode (4) in a electrolytic solution (3), taking the conductive base (1) as an anode, and forming a tantalum dioxide anti-diffusion layer on the conductive base (1) after energizing.