A method for fabricating a thin film transistor includes providing a substrate (100); forming a semiconductor layer (105) over the substrate (100); forming a source-drain metal layer (106) over the semiconductor layer (105); applying one patterning process to the semiconductor layer (105) and the source-drain metal layer (106) to form an active layer (1), a source electrode (2), and a drain electrode (3); forming a gate insulating layer (101) and an interlayer insulating layer (102) that cover the active layer (1), the source electrode (2), and the drain electrode (3); applying a patterning process to the interlayer insulating layer (102) to form a first window (10) in the interlayer insulating layer (102) to expose a portion of the gate insulating layer (101); and forming a gate electrode (4) in the first window (10). An orthogonal projection of the gate electrode (4) on the substrate (100) is in an orthogonal projection of the active layer (1) on the substrate (100).