A method for doping a layer, a thin film transistor and a method for fabricating the thin film transistor. The method comprises: forming a layer to be doped on a substrate by a first patterning process, wherein the layer comprises a first region, a second region and a third region, the first region is arranged in a middle region, the third region is arranged in an edge region, the second region is arranged between the first region and the third region; forming a first blocking layer and a second blocking layer on the layer in this order by a second patterning process, wherein an orthographic projection region of the first blocking layer on the layer exactly covers the first region, and an orthographic projection region of the second blocking layer on the layer exactly covers the first region and the second region; perform a first doping on the layer with an ion beam perpendicular to the substrate, to realize doping of the third region; rotating the substrate by a preset angle in a direction parallel to the ion beam, so that the second blocking layer does not shield the second region, and performing a second doping on the layer with the ion beam.