An active device array substrate has a display area and a peripheral circuit area and further includes a plurality of pixel units, a plurality of signal lines, a plurality of testing pads and a first dielectric layer. The pixel units are arranged in the display area in an array. The signal lines and the testing pads are arranged in the peripheral circuit area. The first dielectric layer covers the testing pads. A testing method of the active device array substrate is that firstly removing a part of the first dielectric layer to expose a testing pad(s) desired to electrically contact with a testing tool. In other words, before the testing, the testing pads are electrically insulated from the exterior to prevent the pixel units from the electrostatic charges damage and thus the circuit stability of the active device array substrate can be improved.