Embodiments of this application provide a data transmission method, a terminal device, and a network side device. Encoding, by a terminal device, a first identifier by using a first code word which is selected from at least one code word by the terminal device; sending, by the terminal device, the encoded first identifier to a network side device; receiving, by the terminal device, a second identifier sent by the network side device; decoding, by the terminal device, the second identifier by using the first code word; when the decoded second identifier is the same as the first identifier, encoding, by the terminal device, subsequent data by using the first code word; and sending, by the terminal device, the encoded subsequent data to the network side device.