A DC uninterruptible power supply system includes plural uninterruptible power supply devices, each of which includes a power cord, a battery module, a voltage detecting circuit, a current detecting circuit, and a control unit. When the voltage detecting circuit detects that a voltage at the power cord is lower than a first preset value, the control unit controls the battery module to output electrical power to the power cord. When the current detecting circuit detects that current of another uninterruptible power supply device is smaller than a second preset value, the control unit controls a switch to permit current flow to the another uninterruptible power supply device.