A UPS with voltage-balancing state detection of batteries includes a processor, a voltage measurement module, a voltage control module, a battery pack and a display unit. Each of the voltage measurement module and the voltage control module is connected to the processor and the battery pack. The processor is connected to the display unit, generates a voltage index value according to voltage values measured by the voltage measurement module and a voltage-balancing state according to the voltage index value and a set of index ranges, controls charging to the battery pack according to the voltage values through the voltage control module, acquires an updated voltage-balancing state of the battery pack, and instructs the display unit to display the voltage-balancing state to prompt users for battery replacement when the voltage-balancing state and the updated voltage-balancing state acquired within a state-holding time are identical.