This application provides a data transmission method, a terminal device, and an access network device. The data transmission method includes: receiving, by a terminal device, information, sent by an access network device, about a carrier corresponding to the terminal device; determining, based on the information about the carrier, a process quantity corresponding to the carrier; and transmitting data based on the process quantity corresponding to the carrier. In this way, the process quantity can be determined accurately and efficiently, thereby improving data transmission reliability.