A data transmission method and apparatus are provided, which includes: determining, by a transmitting device, a long training sequence set; determining at least one receiving device set, where the receiving device set includes at least one receiving device in a communications system; determining a first mapping relationship, where the first mapping relationship is a mapping relationship between the at least one receiving device set and at least one set-use long training sequence in the long training sequence set; determining, according to a target receiving device set and the first mapping relationship, a target set-use long training sequence corresponding to the target receiving device set, and performing encapsulation processing on first data to generate a first data packet, where a long training sequence field in a physical layer of the first data packet carries a first long training sequence symbol.