This application provides a transmission method and a data transmission apparatus. The method includes: sending, by a source access network device, data packets of a QoS flow and a first end marker packet to a first core network user plane device over a first data tunnel, wherein the first end marker packet indicates an end of sending data packets of the QoS flow to the first core network user plane device, and the first data tunnel is corresponding to a first PDU session; sending, by the first core network user plane device, the data packets received from the source access network device and a second end marker packet to a second core network user plane device over a second data tunnel; and sending, by the second core network user plane device, the data packets received from the first core network user plane device to a target access network device.