The present disclosure relates to methods and devices for adjusting a contention window size. One example method includes obtaining, by a first device, at least two hybrid automatic repeat request (HARQ) states corresponding to a first HARQ process identifier, where the at least two HARQ states are different, and the first HARQ process identifier is a HARQ process identifier used when the first device sends data to a second device in a reference time unit, determining, by the first device, a valid HARQ state in the at least two HARQ states, and adjusting a contention window size for a first burst based on the valid HARQ state, where the first burst is later than the reference time unit.