A united converter apparatus and an operating method thereof are disclosed. The united converter apparatus includes a high-voltage charger charging a high-voltage battery, using a commercial power and a low-voltage charger charging a low-voltage battery, using the high-voltage battery. The high-voltage charger includes a voltage level adjustment device boosts a voltage of the commercial power to apply the boosted voltage to the high-voltage battery and drops a high voltage applied from the high-voltage battery to apply the dropped voltage to the low-voltage charger.