The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for calculating and reporting a data available for transmission in a PDCP entity in a dual connectivity system, the method comprising: triggering an event for reporting an amount of data available for transmission in a PDCP (Packet Data Convergence Protocol) entity, wherein the PDCP entity is mapped to two RLC (Radio Link Control) entities and two MAC (Medium Access Control); and reporting the amount of data available for transmission in the PDCP entity as zero to the first BS and the second BS.