This application discloses a method and an apparatus for obtaining an identifier of a terminal device. The method is performed by a gateway node and includes: receiving a first message sent by a terminal device, and sending a second message to a control plane node, where both the first message and the second message carry a temporary identifier of the terminal device; receiving a third message sent by the control plane node, where the third message is sent by the control plane node when the control plane node cannot obtain a permanent identifier of the terminal device; sending a fourth message to the terminal device based on the third message; receiving a fifth message sent by the terminal device, where the fifth message carries the permanent identifier of the terminal device; and sending the permanent identifier to the control plane node based on the fifth message.