A power management method for a server system is provided. At least any one of a power status indication signal and an alert signal from a power supply is detected to judge whether an input voltage is normal. If it is judged that the input voltage is abnormal, a motherboard sends the power status indication signal to a battery backup unit (BBU) to inform the BBU to supply power to the motherboard. If it is judged that the input voltage is abnormal, the motherboard lowers its loading.