The invention discloses a thin film transistor, a preparation method thereof, an array substrate and a display device. The threshold voltage of the active layer is regulated by adding oxygen ions andhydrogen ions into the active layer prepared by metal oxide semiconductor materials. When preparing the thin film transistor, each sub-etching barrier layer is obtained by step-by-step deposition using a plasma enhanced chemical vapor deposition process so as to selectively incorporate oxygen ions and hydrogen ions into the metal oxide semiconductor material when each sub-etching barrier layer isdeposited. Thereby regulating the amount of oxygen ions incorporated into the metal oxide semiconductor material and the amount of hydrogen ions incorporated. In this way, the metal oxide semiconductor material can be doped with oxygen ions and hydrogen ions without increasing the additional doping process, so that the preparation difficulty of the process can be reduced and the preparation cost can be reduced.