A fabrication method of a thin film transistor, a fabrication method of an array substrate, a display panel, and a display device are provided. The fabrication method of the thin film transistor comprises: forming a gate electrode, a gate insulating layer and an oxide active layer; forming an inverted trapezoidal dissolution layer whose cross section is inverted trapezoidal on the oxide active layer, the inverted trapezoidal dissolution layer being soluble in an organic solvent; forming a source/drain layer on the oxide active layer, the gate insulating layer and the inverted trapezoidal dissolution layer, a thickness of the inverted trapezoidal dissolution layer being greater than a thickness of the source/drain layer; and dissolving and removing the inverted trapezoidal dissolution layer with the organic solvent and removing the source/drain layer on the inverted trapezoidal dissolution layer, to form a source electrode and a drain electrode.