Embodiments of this disclosure provide contention window size determining methods and apparatuses. One method includes: sending, by a first device, a data packet that comprises a code block group (CBG), wherein the CBG at least partially overlaps with a first subband and at least partially overlaps with a second subband, receiving, by the first device, a hybrid automatic repeat request-acknowledgement (HARQ-ACK) corresponding to the CBG, and determining, by the first device, a contention window size of the first subband and a contention window size of the second subband based on the HARQ-ACK.