The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting a power headroom reporting in the wireless communication system, the method comprising: generating a PHR MAC CE (Power Headroom Reporting MAC Control Element) to be transmitted to the first BS; and transmitting a power headroom reporting through the PHR MAC CE to the first BS, wherein the PHR MAC CE includes only first PH (Power Headroom) information for activated cells served by the first BS such that second PH information for activated cells served by the second BS is not included.