A rack server system including at least one server and a battery backup unit (BBU) is provided. A power supplier is coupled to the server for converting an input voltage into a first output voltage when the input voltage is normal and for providing the first output voltage to the server. The BBU is coupled to the server and the power supplier for detecting the first output voltage outputted from the power supplier and for providing a second output voltage to the server when the input voltage and/or the first output voltage are abnormal.