A method for fabricating an array substrate, comprising: fabricating a pixel circuit, the pixel circuit including: a driving transistor including a first active medium made of polysilicon, the first active medium having a first grain size; and a switching transistor including a second active medium made of polysilicon, the second active medium having a second grain size larger than the first grain size; and fabricating an organic light-emitting diode coupled to the pixel circuit, wherein fabricating the pixel circuit includes: forming an amorphous silicon layer over a substrate; forming a gate insulating layer over the amorphous silicon layer; forming a preset pattern, an orthogonal projection of the preset pattern on the substrate overlapping an orthogonal projection of the first active medium on the substrate and not overlapping with an orthogonal projection of the second active medium on the substrate; performing laser annealing to turn the amorphous silicon layer into a polysilicon layer; and forming the driving transistor and the switching transistor based on the polysilicon layer and the gate insulating layer, characterized in that the preset pattern is formed over the gate insulating layer, the preset pattern is removed after performing laser annealing to turn the amorphous silicon layer into the polysilicon layer and before forming the driving transistor and the switching transistor based on the polysilicon layer and the gate insulating layer, and removing the preset pattern includes applying a preset etching solution capable of etching the preset pattern and the gate insulating layer to remove the preset pattern and reduce a thickness of a region of the gate insulating layer not covered by the preset pattern. a driving transistor including a first active medium made of polysilicon, the first active medium having a first grain size; and a switching transistor including a second active medium made of polysilicon, the second active medium having a second grain size larger than the first grain size; and fabricating an organic light-emitting diode coupled to the pixel circuit, wherein fabricating the pixel circuit includes: forming an amorphous silicon layer over a substrate; forming a gate insulating layer over the amorphous silicon layer; forming a preset pattern, an orthogonal projection of the preset pattern on the substrate overlapping an orthogonal projection of the first active medium on the substrate and not overlapping with an orthogonal projection of the second active medium on the substrate; performing laser annealing to turn the amorphous silicon layer into a polysilicon layer; and forming the driving transistor and the switching transistor based on the polysilicon layer and the gate insulating layer, characterized in that the preset pattern is formed over the gate insulating layer, the preset pattern is removed after performing laser annealing to turn the amorphous silicon layer into the polysilicon layer and before forming the driving transistor and the switching transistor based on the polysilicon layer and the gate insulating layer, and removing the preset pattern includes applying a preset etching solution capable of etching the preset pattern and the gate insulating layer to remove the preset pattern and reduce a thickness of a region of the gate insulating layer not covered by the preset pattern. forming an amorphous silicon layer over a substrate; forming a gate insulating layer over the amorphous silicon layer; forming a preset pattern, an orthogonal projection of the preset pattern on the substrate overlapping an orthogonal projection of the first active medium on the substrate and not overlapping with an orthogonal projection of the second active medium on the substrate; performing laser annealing to turn the amorphous silicon layer into a polysilicon layer; and forming the driving transistor and the switching transistor based on the polysilicon layer and the gate insulating layer, characterized in that the preset pattern is formed over the gate insulating layer, the preset pattern is removed after performing laser annealing to turn the amorphous silicon layer into the polysilicon layer and before forming the driving transistor and the switching transistor based on the polysilicon layer and the gate insulating layer, and removing the preset pattern includes applying a preset etching solution capable of etching the preset pattern and the gate insulating layer to remove the preset pattern and reduce a thickness of a region of the gate insulating layer not covered by the preset pattern.