A GOA substrate includes N shift register units. The first stage shift register unit outputs a scan signal pulse based on a first clock signal and a start signal. The last stage shift register unit outputs a scan signal pulse based on Mth clock signal and the start signal. The start signal has a pulse width starting from a falling edge of the Mth clock signal of the last stage shift register unit when scanning a first frame, and ending at a rising edge of the first clock signal of the first stage shift register unit when scanning a second frame. Since the first and last shift register units are used to drive a scan signal pulse based on the start signal, the present invention reduces the number of wires needed to transmit start signals and simplifies the complexity of the layout design.