An uninterruptible power system and an operation method thereof are provided. The uninterruptible power system comprises a DC-AC conversion circuit, a plurality of switches, a plurality of sensing units, a plurality of output ports and a control unit. Each output port is electrically coupled to an output terminal of the DC-AC conversion circuit sequentially through one of the sensing units and one of the switches. The control unit is configured to define members of at least one group from the output ports according to a system setting, and define which members of each group are non-critical output ports according to the system setting. The control unit is further configured to set, according to the system setting, at least one condition for all non-critical output ports in each group to simultaneously stop supplying power, and to accordingly control the operations of the corresponding switches.