Embodiments of this application provide a security protection method, a device, and a system, to improve data transmission security. The method includes: determining, by a terminal, a session management network element, or a mobility management network element, whether a security protection policy determined by an access network device is consistent with a user plane security policy delivered by the session management network element to the access network device; and if the security protection policy determined by the access network device is inconsistent with the user plane security policy delivered by the session management network element to the access network device, performing processing according to a preset policy.