A thin film transistor, a manufacturing method for an array substrate, the array substrate, and a display device are provided. The manufacturing method for a thin film transistor includes: forming a semiconductor layer; performing a modification treatment on a surface layer of a region of the semiconductor layer, so that the region of the semiconductor layer has a portion in a first direction perpendicular to the semiconductor layer formed as an etching blocking layer, portions of the semiconductor layer on both sides of the etching blocking layer in a second direction parallel to a surface of the semiconductor layer remaining unmodified; and forming a source electrode and a drain electrode on the semiconductor layer, the source electrode and the drain electrode being formed on both sides of a center line of the region perpendicular to the second direction, and spaced from each other in the second direction.