At present, it is very convenient to use mobile terminal devices because of the employment of a USB interface as a power interface by chargers for most of mobile terminal devices such as mobile phones, cameras and PAD and due to the easiness of getting a standard USB master device which is capable of charging these mobile terminal devices. However, as the current provided by a standard USB master device is small, for example, the current provided by a standard USB master device with a USB2.0 interface is 500 mA, and that provided by a standard USB master device with a USB1.1 interface is 250 mA, it takes a long time to charge a mobile terminal using a standard USB master device, resulting in poor user experience. Some existing portable power sources with a USB interface are capable of supplying a larger charging current, for example, a current of 800 mA, 1 A or even 1.5 A, than a standard USB master device. For existing USB master devices having a large-current charging capacity, as a USB slave device, that is, a mobile terminal, is incapable of automatically changing the magnitude of a charging current according to the power supply capacity of such a USB master device, the charging efficiency is low. For customers, it is desirable that a mobile terminal is capable of selecting a charging current as needed while providing a data communication service and that a plurality of charging modes are selectable for the convenient and rapid charging of a mobile terminal.