An expansion circuit for a server system includes a first output terminal, a second output terminal, a switch circuit and a detection circuit. The first output terminal receives a first voltage and providing the first voltage to a first hard disk drive group. The second output terminal receives the first voltage and provides the first voltage to a second hard disk drive group. The switch circuit is connected between the first and the second output terminals. The detection circuit detects a number of the at least one server which is electrically connected to the expansion circuit. When two servers are electrically connected to the expansion circuit, the detection circuit switches off the switch circuit. When a server is electrically connected to the expansion circuit, the detection circuit switches on the switch circuit.