The invention discloses a manufacturing method of a thin film transistor and a transistor manufactured by the method, wherein the method is characterized by comprising the following steps: sequentially forming a surface covering layer, a polycrystalline silicon island active layer, a grid insulating layer, a grid electrode conducting layer and an auxiliary layer on a baseplate, photoetching the auxiliary layer to form an LDD forming layer, and forming a lightly doped drain electrode region, a heavily doped source electrode, a heavily doped drain electrode and a channel on the polycrystalline silicon island active layer by utilizing the action of the LDD forming layer and through only once doped ion implantation. The method of the invention only needs once ion implantation, simplifies the manufacturing working procedures and lowers the manufacturing cost; the thin film transistor which is manufactured by adopting the method and provided with a lightly doped drain electrode has the advantages of small electrode area and large numerical aperture.