A thin film transistor and a preparation method thereof, an array substrate and a display apparatus are provided. The preparation method includes an operation of forming a low temperature poly silicon active layer; a substrate has a first region and a second region; and the step includes: forming a buffer layer on the first region and the second region of the substrate, the buffer layer having a thickness at a portion corresponding to the first region greater than that at a portion corresponding to the second region; or, forming the buffer layer on the first region of the substrate; forming an amorphous silicon layer on the buffer layer; performing laser crystallization processing on the amorphous silicon layer so as to convert the amorphous silicon layer into a poly silicon layer; and removing the poly silicon layer on the second region, and forming the low temperature poly silicon active layer on the first region.