A thin film transistor, a method for fabricating the same, a display substrate, and a display device are disclosed. The thin film transistor includes a gate, a source, a drain, and an active layer. Forming the active layer includes: forming a pattern comprising a thermal insulation layer; forming a pattern comprising an amorphous silicon layer on the thermal insulation layer, wherein the pattern comprising the amorphous silicon layer includes a first portion on the thermal insulation layer and a second portion extending beyond the thermal insulation layer; and treating the pattern comprising the amorphous silicon layer with a laser annealing process, so that the amorphous silicon layer grows grain in a direction from the second portion to the first portion to form the active layer from polycrystalline silicon.