A method for fabricating a thin film transistor, comprising: 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 a patterning process, wherein only one mask is used in the patterning process, the forming the pattern of the light shielding layer, the insulating layer, and the semiconductor layer in the patterning process comprises: forming photoresist in a first region of the semiconductor layer; etching the semiconductor layer to remove the semiconductor layer outside the first region; etching the insulating layer for a first time to remove the insulating layer outside the first region; treating the photoresist, so that the photoresist has a width which is smaller than a width of the first region, the treated photoresist corresponding to a second region; etching the light shielding layer and the semiconductor layer, to remove the light shielding layer outside the first region and the semiconductor layer outside the second region, and etching the insulating layer for a second time, so that the insulating layer has a width which is larger than a width of the second region and smaller than the width of the first region. 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 a patterning process, wherein only one mask is used in the patterning process, the forming the pattern of the light shielding layer, the insulating layer, and the semiconductor layer in the patterning process comprises: forming photoresist in a first region of the semiconductor layer; etching the semiconductor layer to remove the semiconductor layer outside the first region; etching the insulating layer for a first time to remove the insulating layer outside the first region; treating the photoresist, so that the photoresist has a width which is smaller than a width of the first region, the treated photoresist corresponding to a second region; etching the light shielding layer and the semiconductor layer, to remove the light shielding layer outside the first region and the semiconductor layer outside the second region, and etching the insulating layer for a second time, so that the insulating layer has a width which is larger than a width of the second region and smaller than the width of the first region. forming photoresist in a first region of the semiconductor layer; etching the semiconductor layer to remove the semiconductor layer outside the first region; etching the insulating layer for a first time to remove the insulating layer outside the first region; treating the photoresist, so that the photoresist has a width which is smaller than a width of the first region, the treated photoresist corresponding to a second region; etching the light shielding layer and the semiconductor layer, to remove the light shielding layer outside the first region and the semiconductor layer outside the second region, and etching the insulating layer for a second time, so that the insulating layer has a width which is larger than a width of the second region and smaller than the width of the first region.