A low temperature poly-silicon thin film transistor and its manufacturing method, an array substrate and a display device are provided. The method comprises: forming a poly-silicon film on a base substrate (1) and patterning the poly-silicon film to form an active layer (3); forming a gate insulation layer (4) on the active layer (3) and performing hydrogenation process to the gate insulation layer (4) and the active layer (3). By this method, the diffusion distance of hydrogen is largely shortened, the time for the hydrogenation process is reduced, and thus the process cost for the thin film transistor is largely lowered down.