A hold-up time extension controlling device for a power supply, that has a power factor correction circuit, a hold-up time extension circuit, a hold-up time controlling circuit, a storage capacitor and a DC to DC converter. The hold-up time extension controlling device has a switch and an auxiliary capacitor. When an AC power source is normal, the switch is controlled to turn on according to the power factor correction circuit to make the auxiliary capacitor connecting to the storage capacitor in parallel to averagely have a ripple current. When the AC power source is interrupted, the switch is controlled to turn off. At the time, the hold-up time extension circuit obtains a storage power of the auxiliary capacitor and then output to the DC to DC converter to keep the voltage of the storage capacitor at a preset voltage for a long time.