A method for determining a contention window size in clear channel assessment, including: determining, at least one user equipment first UE that occupies an uplink subframe on an unlicensed carrier and that is in at least one UE scheduled in a reference subframe, where the uplink subframe is an uplink subframe in which at least one UE scheduled by the base station in the reference subframe feeds back a hybrid automatic repeat request HARQ status for the reference subframe; and determining, a CWS in CCA for downlink transmission based on a HARQ status of the at least one first UE that occupies the uplink subframe.