A thin film transistor, a method for fabricating the same, a display substrate, and a display device are disclosed. The method comprises: forming in sequence a light shielding layer, an insulating layer, and a semiconductor layer; and forming a pattern of the light shielding layer, the insulating layer, and the semiconductor layer in one patterning process. A polycrystalline silicon layer can be formed into an active layer and an amorphous silicon layer into the light shielding layer, by using only one mask. The number of masking processes is reduced by one, which simplifies a fabricating process of the thin film transistor.