A power-supply expansion system includes a primary power-supply unit for providing a main power supply, a secondary power-supply unit for providing an auxiliary power supply, a power unit having a first and a second input terminal and an output terminal, and a control unit connected to the primary and the secondary power-supply unit. The control unit, based on a load value of the primary power-supply unit, selectively performs a power-supply expansion process for the secondary power-supply unit to feed the auxiliary power supply to the second input terminal. In the power-supply expansion process, the first input terminal receives the main power supply and the second input terminal receives the auxiliary power supply, and the power unit integrates the main and the auxiliary power supply into an output power supply, which is output via the output terminal. A power-supply expansion method applicable to the power-supply expansion system is also disclosed.