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 wireless communication system, the method comprising: calculating multiple Power Headroom (PH) values per an active cell; selecting one PH value among the multiple PH values for setting a value of a PH field of a corresponding cell to the selected PH value; generating a Power Headroom Reporting (PHR) Medium Access Control (MAC) Control Element (CE) including the PH field set to the selected PH value; and transmitting a MAC Protocol Data Unit (PDU) including the generated PHR MAC CE to a network.