A power adapter includes a power converting circuit, a current detecting circuit, and a controlling unit. A charging voltage with a constant voltage level is provided by the hub through the connecting ports. The current detecting circuit is used for detecting a load current and outputting a corresponding current detecting signal. The load current is transmitted from the power converting circuit to the hub through the power cable. A look-up table is stored in the controlling unit for recording a relationship between the load current and a voltage drop across an impedance of the power cable. According to the current detecting signal and the look-up table, the controlling unit issues a feedback signal to the power converting circuit. According to the feedback signal, a level of the output voltage is dynamically adjusted to be equal to the sum of the voltage drop and the charging voltage.