A method for adjusting a system performance of a portable electronic device is disclosed. The portable electronic device is connected to a power adapter for obtaining external power supply. The method comprises the steps of: receiving a voltage signal outputted from the power adapter; determining whether to execute an over current protection by the power adapter; calculating a recovering time of the voltage signal if the over current protection is executed by the power adapter, wherein the recovering time is a time period from stopping an output of the voltage signal to restarting the output thereof; and determining an output power of the power adapter to adjust the system performance of the portable electronic device according to the recovering time.