In this paper, ATLAS 2D device simulator of SILVACO was used for device simulation of invertedstaggered thin film transistor using amorphous indium gallium zinc oxide as active layer (a-IGZO-TFT) with double active layers, based on the density of states (DOS) model of amorphous material. The change of device performance induced by the thickness variation of each active layer was studied, and the interface between double active layers was analyzed. The best performance was found when the interface was near the edge of the channel, by optimizing the thickness of each active layers, the high performance device of threshold voltage (V_th) = – 0.89 V, sub-threshold swing (SS)= 0.27, on/off current ratio (ION/IOFF) = 6.98 × 10¹⁴ was obtained.