A power headroom reporting (PHR) processing method in carrier aggregation system and a device thereof are provided. The PHR processing method includes the following steps: user equipment (UE) acquires a pathloss change corresponding to an uplink carrier which is in an activation state at present; the UE determines whether a PHR is to be triggered based on the pathloss change; if the PHR is to be triggered, the UE acquires power headroom (PH) information to be reported and reports the PH information.