A data transmission method, applied to a first terminal device, includes receiving a first message from a second terminal device, determining a first session parameter based on the first message, determining a first session based on the first session parameter, receiving first data from the second terminal device, and sending the first data to a network device using the first session. The first terminal device may determine, based on the first message from the second terminal device, the first session parameter that meets a service requirement of the second terminal device, determine, based on the first session parameter, the first session that meets the service requirement of the second terminal device, and forward data of the second terminal device to the network device using the first session.