A flat display panel and an active device array substrate and a light-on testing method thereof are provided. The active device array substrate comprises a plurality of first pixel units, a plurality of second pixel units, a first light-on testing circuit, a second light-on testing circuit and a plurality of sets of signal lines. The first light-on testing circuit and the second light-on testing circuit disposed in a peripheral circuit region of the active device array substrate are electrically connected with the first pixel units and the second pixel units disposed in a display region of the active device array substrate respectively. Each two adjacent sets of signal lines disposed in the peripheral circuit region of the active device array substrate are alternatively electrically connected to the first pixel units or the second pixel units respectively. When a testing signal inputted to first/second pixel units through the first/second light-on testing circuit results in any second/first pixel units being lighted-on, there are at least two adjacent sets of signal lines shorted to each other.