The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for transmitting lossless data packet based on QoS framework in wireless communication system, the method comprising: starting a timer for a first QoS flow when a DRB which is mapped to the first QoS flow is changed from a first DRB to a second DRB; delivering packets of the first QoS flow received from the first DRB to an upper layer while the timer is running; when the timer expires, starting to deliver the packets of the first QoS flow received from the second DRB while stopping the delivering packets of the first QoS flow received from the first DRB.