An active device array substrate including a substrate, scan lines, data lines, pixels, a bus line and voltage pull-down circuits is provided. The pixels disposed on intersections of the scan lines and the data lines are arranged in array on the substrate and are electrically connected to the scan lines and the data lines. Each of the voltage pull-down circuits including a transistor and an electrostatic discharge protection device is electrically connected between the scan line and the bus line correspondingly. Each transistor includes a source, a drain, and a gate electrically connected to a next scan line. Each gate is electrically connected to the scan line, the source, the drain and the bus line correspondingly through the electrostatic discharge protection device. The electrostatic discharge protection of the active device array substrate is enhanced effectively by the electrostatic discharge protection device.