Embodiments of this application provide a contention window management method applied to an unlicensed band. The method includes sending, by a sending device, one or more data packets to one or more receiving devices in one reference time unit or a plurality of reference time units, where the one or more data packets occupy a first bandwidth. The method further includes receiving, by the sending device from the one or more receiving devices, a hybrid automatic repeat request HARQ or HARQs for the one or more data packets. The sending device determines a contention window (CW) size of a second bandwidth with reference to the HARQ or HARQs. The CW update based on HARQ feedback improves communication efficiency.