The present invention relates to a wireless access system that supports an unlicensed band, and provides various methods for adjusting a contention window size and devices for supporting the same. As an embodiment of the present invention, a method for adjusting a contention window size (CWS) in a wireless access system supporting an unlicensed band may comprise the steps of: performing a channel access procedure; when, as a result of performing the channel access procedure, it is determined that a serving cell (Scell) in the unlicensed band is in an idle state, transmitting a transmission burst including a downlink signal; receiving HARQ-ACK information corresponding to the transmission burst; and adjusting the CWS on the basis of the HARQ-ACK information corresponding to the first subframe of the transmission burst.