The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for power headroom reporting in the wireless communication system, the method comprising: determining a first weight factor for the first BS and a second weigh factor for the second BS if a sum of a power headroom (PH) for the first BS or a PH for the second BS is more than a PH of activated cells of the UE; reducing a size of the PH for the first BS by using the first weight factor; and reducing a size of PH for the second BS by using the second weight factor.