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 . <EOS>