A wireless network system includes a coordinator, a first device, and a second device. The coordinator transmits a first response to the first device to approve a first request message received from the first device; broadcast a first beacon, indicating that the first device is in a power saving mode, after transmitting the first response; transmit a second response to the second device to approve a second request message received from the second device; broadcast a second beacon, indicating that the first device needs to change a power management mode from the power saving mode to a normal mode, after transmitting the second response; transmit a third response to the first device to approve a third request message received from the first device; and broadcast a third beacon, indicating that the first device is in the normal mode, after transmitting the third response.