The lifetime of an organic light emitting display (OLED) generally depends on many aspects. Among them, a driving scheme has recently been considered to be one of the most important factors for improving the performances of OLEDs. It is known that an ac driving mode combined with a reversed-bias voltage improves the lifetime of an OLED. In this brief we present a circuit to understand ac driving mode in active matrix OLED (AMOLED), then simulate this pixel circuit and a 1*4 matrix. By simulation we observed that OLED is in an ac driving mode combined with a reversed-bias voltage. In the end a circuit design for understanding ac driving mode was presented.