A charging device and a control method thereof are provided. The charging device includes an adapter, an energy storage unit and a charging module. The adapter provides an adapter current. The charging module receives the adapter current and provides a first charging current to charge the energy storage unit, or an output current to charge an electronic device. When the input current is larger than a first preset current and the first charging current is equal to or less than a third preset current, the charging module operates in a boost mode, such that the energy storage unit provides a second charging current to the charging module. When the second charging current is larger than the third preset current, the charging module enters a buck mode such that the energy storage unit is charged by the first charging current.