Disclosed, in the present invention, are a method for transmitting, by a base station, a downlink signal in a wireless communication system supporting an unlicensed band, and a device for supporting the same. More particularly, in accordance with one embodiment of the present invention, when acknowledgment timing of each terminal (for example, an interval length from a time point of receiving downlink data to a time point of transmitting acknowledgment information for the downlink data) is set flexibly (or independently), a base station may determine a reference downlink resource for contention window size (CWS) adjustment by considering the acknowledgement timing and perform downlink signal transmission by adjusting CWS on the basis of the acknowledgment information received for the reference downlink resource.