The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for triggering power headroom reporting in a dual connectivity system, the method comprising: configuring a plurality of Medium Access Control (MAC) entities, each MAC entity corresponding to each enhanced-NodeB (eNB); occurring an event for triggering power headroom reporting (PHR) in a MAC entity of the plurality of MAC entities; triggering the PHR in the plurality of MAC entities including the MAC entity if a type of the event is a first type event; and triggering the PHR in the MAC entity if the type of the event is a second type event.