An active device array substrate and its fabricating method are provided. According to the subject invention, the elements of an array substrate such as the thin film transistors, gate lines, gate pads, data lines, data pads and storage electrodes, are provided by forming a patterned first metal layer, an insulating layer, a patterned semiconductor layer and a patterned metal multilayer. Furthermore, the subject invention uses the means of selectively etching certain layers. Using the aforesaid means, the array substrate of the subject invention has some layers with under-cut structures, and thus, the number of the time-consuming and complicated mask etching process involved in the production of an array substrate can be reduced. The subject invention provides a relatively simple and time-saving method for producing an array substrate.