An operation method and device for an integrated modem having a plurality of modems is provided. The method includes connecting the integrated modem to a mobile terminal, supplying, by the mobile terminal, power to each modem of the integrated modem, performing booting each modem to maintain a ready-on state, and controlling to operate a modem selected according to an input signal from an input unit while terminating supplying of power to the non-selected modem or maintaining the booting completion state.