A power supply controlling system is configured to supply power to a load and includes a first power supply device having a first outputting voltage and current to the load, a second power supply device having a second outputting voltage and current to the load, and a buck converter which is selectively connected to the first power supply device or the second power supply device first and is then connected in parallel with the power supply device without converter and the load. The buck converter is selectively connected to the power supply device with a higher voltage, and controls the output voltage of the converter to make it conform to the voltage of the other power supply device, so as to connect the two power supply devices in parallel and output current to the load.