A USB hub includes an upstream port; a first USB device control unit for performing packet format conversion; an FIFO circuit for storing data outputted from the first host; a second USB device control unit for performing packet format conversion; and a plurality of downstream ports. When any one of the downstream ports is coupled to the mobile device, if the first host commands the mobile device to switch to a host role from a device role, the mobile device temporarily disconnects from the USB hub, and the first host commands the USB hub to change an internal routing path. After the mobile device switches to the host role, the first host still performs the host role, and the first USB device control unit performs the device role; the mobile device performs the host role, and the second USB device control unit performs the device role.