A method of manufacturing a thin film transistor includes forming a semiconductor layer on a base substrate; forming a gate electrode on the semiconductor layer; forming a shield on the gate electrode, wherein a perpendicular projection of the shield onto the base substrate covers a first source portion of the source region and a first drain portion of the drain region; and performing ion implantation to the semiconductor layer by using the shield as a mask, so as to form a first doped region in the first source portion and in the first drain portion, and to form a second doped region in a second source portion of the source region that is not covered by the perpendicular projection of the shield and in a second drain portion of the drain region that is not covered by the perpendicular projection of the shield.