The disclosure provides data transmission methods and apparatuses. One example method includes when the terminal device needs to perform the cross-network movement, the terminal device sends a message to a network element of a network accessed by the terminal device, to notify that the terminal device is to perform the cross-network movement from a source network to a target network. Therefore, during the movement of the terminal device, a network side pauses transmission of user plane data of the terminal device, and buffers data that is not sent before the terminal device performs the cross-network movement. After accessing the target network, the network side sends the buffered data to the terminal device.