In various embodiments of the disclosed subject matter, a method for forming a thin film transistor (TFT), a related TFT, array substrate, and display apparatus are provided. The method comprises: forming a pattern of an active layer on a base substrate and insulated from a gate electrode; forming a first initial ohmic contacting layer and a second initial ohmic contacting layer on the active layer; forming a source electrode on the first initial ohmic contacting layer, and a drain electrode on the second initial ohmic contacting layer; and performing a heating treatment to the base substrate having the source electrode and the drain electrode thereon, such that metal atoms in the source electrode diffuse to the first initial ohmic contacting layer to form a first ohmic contacting layer, and metal atoms in the drain electrode diffuse to the second initial ohmic contacting layer to form a second ohmic contacting layer.