A short message transmission method, comprising: receiving, by a communications device, uplink data and indication information from a terminal device, wherein the indication information is used to indicate that the uplink data belongs to a short message service; determining, by the communications device, identification information, wherein the identification information is used to identify a short message function entity device that processes the short message service; and sending, by the communications device, the uplink data to the short message function entity based on the indication information and the identification information, the method further comprising: sending, by the communications device, a first request message to the short message function entity device; and receiving, by the communications device, a first response message from the short message function entity device, wherein the first response message is used to indicate that the short message function entity device successfully registers the short message service, wherein the determining, by the communications device, identification information comprises: obtaining, by the communications device, an association between the identification information and the terminal device, wherein before the sending, by the communications device, first request message to the short message function entity device, characterized in that the method further comprises: receiving, by the communications device, an attach request message from the terminal device, wherein the attach request message carries capability information, and the capability information is used to trigger the communications device to send the first request message; and after receiving the uplink data from the terminal device, determining, by the communications device, that the communications device does not have the identification information; and receiving, by the communications device, a notification message from a subscriber data management SDM device, wherein the notification message carries a cause value: a mobile terminated short message service, MT-SMS, and the cause value is used to trigger the communications device to send the first request message. receiving, by a communications device, uplink data and indication information from a terminal device, wherein the indication information is used to indicate that the uplink data belongs to a short message service; determining, by the communications device, identification information, wherein the identification information is used to identify a short message function entity device that processes the short message service; and sending, by the communications device, the uplink data to the short message function entity based on the indication information and the identification information, the method further comprising: sending, by the communications device, a first request message to the short message function entity device; and receiving, by the communications device, a first response message from the short message function entity device, wherein the first response message is used to indicate that the short message function entity device successfully registers the short message service, wherein sending, by the communications device, a first request message to the short message function entity device; and receiving, by the communications device, a first response message from the short message function entity device, wherein the first response message is used to indicate that the short message function entity device successfully registers the short message service, wherein the determining, by the communications device, identification information comprises: obtaining, by the communications device, an association between the identification information and the terminal device, wherein before the sending, by the communications device, first request message to the short message function entity device, characterized in that obtaining, by the communications device, an association between the identification information and the terminal device, wherein before the sending, by the communications device, first request message to the short message function entity device, characterized in that the method further comprises: receiving, by the communications device, an attach request message from the terminal device, wherein the attach request message carries capability information, and the capability information is used to trigger the communications device to send the first request message; and after receiving the uplink data from the terminal device, determining, by the communications device, that the communications device does not have the identification information; and receiving, by the communications device, an attach request message from the terminal device, wherein the attach request message carries capability information, and the capability information is used to trigger the communications device to send the first request message; and after receiving the uplink data from the terminal device, determining, by the communications device, that the communications device does not have the identification information; and receiving, by the communications device, a notification message from a subscriber data management SDM device, wherein the notification message carries a cause value: a mobile terminated short message service, MT-SMS, and the cause value is used to trigger the communications device to send the first request message.