The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for triggering and reporting a buffer status in the wireless communication system, the method comprising: configuring a plurality of logical channel sets comprising a first logical channel set transmitting data to the first BS and a second logical channel set transmitting data to the second BS, wherein the first and the second logical channel sets respectively comprise one or more logical channels; receiving data for a first logical channel belonging to the first logical channel set; and triggering a buffer status reporting to the first BS, if the first logical channel has highest priority among logical channels for which data are available for transmission in the first logical channel set.