Embodiments of the present invention relate to the communications field, and provide a data transmission method, a sending device, and a receiving device. Transmission requirements of different subbands can be maximally satisfied, and a system throughput can be improved. The method includes: determining, by a sending device, rank indicators of J (an integer greater than or equal to 1) subbands corresponding to a to-be-transmitted channel, where the rank indicators of the J subbands are not totally the same; determining, by the sending device based on a rank indicator of each of the J subbands, a transmission block size.