A data transmission method to implement flexible switching of data transmission between different interfaces, includes: A first terminal obtains a quality of service flow identifier of the first terminal and a QoS parameter of a quality of service flow, where there is a correspondence between the quality of service flow identifier and the QoS parameter of the quality of service flow, and determines, based on the QoS parameter, a logical channel used by the quality of service flow on a PC5 interface. The first terminal maps the quality of service flow to the determined logical channel, and sends the quality of service flow to the second terminal.