A power supply conversion system receives an external power source to supply power to a load. The power supply conversion system includes at least one main power apparatus, at least one auxiliary power apparatus, a main switch, an auxiliary switch, and a control unit. The control unit turns on the main switch to restore the external power source when the control unit detects that the external power source is normally restored, and jointly supply power to the load with the auxiliary power apparatus. Especially, the output voltage of the main power apparatus is greater than the output voltage of the auxiliary power apparatus. In addition, the control unit disconnects the auxiliary power apparatus supplying power to the load when the control unit detects that the main power apparatus completely supplies power to the load.