A backup power supplying device having programmable current-balancing control includes at least two power modules connected in parallel. The power module includes a power converter, a current sensing component, a potential tuner, a microprocessor, a current-balancing control circuit and an output voltage controller. The current sensing component senses an output current of the power converter to generate a current sensing signal. The microprocessor controls the potential tuner to generate a tuning signal, and receives a mode signal to control the power module to operate in a power supply or sleep mode. The current-balancing control circuit receives the current sensing signal, the tuning signal and the mode signal. When the power module operates in the sleep mode, an output voltage of the power converter is a sleep voltage; a voltage level of the sleep voltage is lower than a voltage level of a supply voltage by a predetermined voltage value.