Provided are a poly-silicon thin film transistor (TFT), a poly-silicon array substrate and a preparing method thereof, and a display device for solving the problems of excessive mask plates, complicated process and high costs in a conventional technology. The method of preparing the poly-silicon TFT comprising a doped region comprises steps: forming a poly-silicon layer on a substrate, forming an active layer by a patterning process; forming a first insulating layer; forming, by a patterning process, via holes exposing the active layer, the source electrode and the drain electrode being connected through the via holes to the active layer; doping the active layer through the via holes by a doping process to form a doped region; forming a source-drain metal layer, and forming the source electrode and the drain electrode by a patterning process.