An interface-adapting device includes an OTG (on-the-go) adapting interface and a voltage-dividing module. The OTG adapting interface is configured to connect an electronic device to an expanding device or to an external power source. The voltage-dividing module is configured to receive an input voltage of the electronic device. When the OTG adapting interface is configured to connect the electronic device to the expanding device, the voltage-dividing module transmits a ground voltage to the electronic device; when the OTG adapting interface is configured to connect the electronic device to the external power source, the voltage-dividing module transmits a divided voltage to the electronic device, so that the external power source is configured to charge the electronic device.