Embodiments of this application relate to a data transmission method, a terminal device, and a network device. The method includes: First, a terminal device receives indication information from a PCF network element, and the indication information indicating data network access identifier information corresponding to an application; and then the terminal device determines, based on the indication information, a PDU session for transmitting data of the application. In this way, the terminal device may select a same PDU session for applications corresponding to same data network access identifier information, so that data transmission efficiency can be improved.